I have mixed feelings on things like this. The "nice guy" in me wants to jump and say yes. The business/reality guy kicks in and I say "sorry, but I just can't."
It depends where you are in your career as well as what this "potential" is with the client. I had a show one time where I was looking for people to help out for a MS benefit. The sound compnay I used....the owner owned a music store. I told him if he bailed me out that I'd give him all my business as well as send all my friends to him. He didn't truly believe me, but he took me at my word and sure enough, I spent insane amounts of money for the next 15 years and used his store exclusively until a new owner took over.
As far as the "where you are in your career" part, what I mean by that is, I've done free gigs from the time I got involved in music for the first 15-20 years. At this stage of the game, I sincerely feel my services are worth something as well as my time. Every 5-6 hours I give up for something free, it takes food off of my table. I could master an entire album in that time or mix a project for a client. I often feel people take advantage of our good nature. In an economy like this, there has to be a really good reason for me to do something for free when I have work piled up over here that can get me real money.
The other side of the coin is, it's always best to do what you feel is right if you feel good about it in your heart. I walk away from "potential clients" every day man. I get guys asking me to master 30 seconds of a project to see if they want to use my services. They just do not understand that it takes me the same amount of time to master a 30 second clip as it does a full song. In that amount of time while I'm hoping this "potential customer" becomes a REAL one, I have jobs waiting around where people have either paid in full before I even started or they have given me half down to start.
You just have to look at things realistically. What if you bring your gear out for this thing and something happens to it? Granted, we are in a situation like this even if someone pays you...but at least you have money to pay for repairs that don't totally come out of your pocket. Now if it were a real customer already, I would try my best to do something for them. But for potentials....people are evil today brother. They will take whatever they can from you while they promise you things and just disappear.
I've actually done loads of things for free for people my entire life. Some I still do today when I feel it's the right thing to do. However, none of the things I ever did really got me business. It got me labeled as "a great guy" but that's about it. Being a great guy doesn't give me the coin to pay the bills per month I need to survive owning a home, two studios, and all that goes with running them. But....sometimes if you feel good about it, you take the shot depending on your situation.
If you're financially secure and want to do it just because, go for it. I've been there too. Sometimes it's nice just to be kind and say "yes". Other times, if you have paying clients waiting for you where you could make money, well, it may not be a good move not to mention, "current clients" will always hold precidence over "potentials" to me. There are just too many people with larceny in their hearts in today's world. Times are totally different unfortunately.
-Danny