Never!  is the correct answer.  When do we do freebies?  Not at anytime because there is no such reality.  I know I'll get tons of debate on this point but let me qualify my response in this way;  I recall vividly an adage given to me as a 6th grade youth from a zealous sixth grade teacher named Mr. Christoff while growing up in Youngstown Ohio.  What he said was {"Theres no such thing as free!  Free is an acronym (F.R.E.E.) which stands for the phrase 'For reasons explained elsewhere'} in other words theres always a hidden agenda! Free is never a reality.  Working for free as an artist or a musician is sinful to say the least.  All the way around any way you look at it; Anyone asking you for free work harbors ill concieved motivations.   So why do we do it?  Why do we find ourselves cheapening our precious contributions to culture and society, and worse than this why do patrons exhibit great desire for something of significant value free of charge without obligation. Market place manipulation of public thinking and perception is a large culprit but I'll hit on that huge subject another time.  The creative mind delivers goods and services just as  Doctors, lawyers, scientists, engineers and many other professional servants and don't you forget it.   
   The analogy of this thing 'free' fused in my head at an early age along with other book learned principles dealing with matters such as  Fair negotiations and Balanced judgments. The conclusion of the study is this:  Any transaction that does not benefit all who are involved in some positive fashion is not a good deal at all and is best abandoned or reworked for corrections.  My focus here is on the effects such activities have on the crafters and developers of intellectual properties not the brokers and plagerists (they have no personal attachment to the activity of developing the goods,) but we artists and musicians who work successfully from scratch to create goods have a vested interest in the success of our crafts.  Here are a couple of simple illustrations that I believe will hone in ones focus to smell the rats, the good cheese from bad cheese and know how to say 'no thankyou' and please come correct or not at all.   patience is definitly a virtue in negotiating; use it wisely if pay is not involved. 

     "Its a charitible function and everybody is donating their time. "  get the details before a yes answer.  Research the charity and the handlers, no matter how big or small the answers are upfront or theres not much organization, its a clear signal to beware.  The participants have varied interests whether declared or not;  makes no difference who is onboard when you have problems or losses after the deal. None of those negotiators will be there for you when its your turn to use the same devil phrase: "are you with us or not".  A well organized business machine works the tax laws also and perhaps you're not that big yet.  If there is a cover charge, ticket sales. condiment and or swag sales then money is being made to recover expenses for more than just the charitible cause.  You have expenses as well!  Preparation, set up, travel, exibition time etc.  The impromptu sale of your merchandise is important as well so negotiate the option to maximize your activity, discussion of the money trails are important. 'if its win win then we can begin.'  Participation in large charitible events can work well however they can also be taxing of your energy and time. Do you remember ever sitting in a doctors lobby for a quick visit and the receptionists telling you "no charge for the visit?"  Rare occurence eh?  A balanced trade off in the negotiation will lead to increases in your acceptance at what you do if nothing else.  If its just a gamble,  calculate your risk and the investment.  The last act on a stage after a worn audience can stir up mixed reviews and send some wrong signals.  Ensure your amount of stage time and the time of your apperance with a written instrument whenever possible, (ie. contract, working memo etc.).  A charitible function with "booth rentals" for the participants isn't a charity or a clear fundraiser.  A battle of the band charity event urging payment of an entry fee is a hustle.  You have talent and skills the promotors need this to pull off their agendas.  Nobody calls the bug exterminator and tells him to pay for entering the house to get rid of the roaches and further more to add insult to the injury tell the exterminator "its going to help him in his business" to do so, and if you wish we'll let you pass out stickers only.   Absurd my friends.  But we do it.  I'm simply saying think twice before you do things like this.  Many fall ignorantly for these 19th and 20th century tactics which are used to this day by the talent hustlers.  Rule:  Never pay to give your good thing away.  Another example:  do the design work or show me what you can do here and then we'll talk about you doing other work for me later.  NO!  Who calls the plummer and says fix this leak and if you do good work we'll talk about paying you next time.  Nobody!  or tell the surgeon voluntarily fix my heart irregularities and if you do good work I'll pay for doing my kidneys.  Request what would be helpful to you as well,  for example, permission to promote your display or charge for the public entry to view the exhibition, or perhaps material and supply supports, set up the tip jar when you do a presentation and make sure that its understood that this is a short promo and the next time is pay at your price.  Never something for nothing,.   Barter and exchange is healthy when fairly administered.  If you get a bad feeling in the beginning theres a reason for it and don't feel bad to walk away.  Certainly there will be other offers and if nothing else you leave with a cheap brag right to go on your hype sheet that you were propositioned for the deal (LOL).  As artsans we don't do freebies.  Goods and services to the market place is taught in schools of higher learning everywhere.  Lets be informed for our successes.  We are scientists doctors and lawyers at what we do creatively.  The politics stinks being a salesman sucks but thats because theres not enough political types and sales folk that respect what we actually do.  Take control, study your craft and learn the art of the deal as a creative mind.  'nuff said 

J. Julio Veal