I don't accept tips during restaurant walkaround. I politely tell the person offering the money something like this:
"I appreciate the offer, but I am paid by the restaurant. Could you maybe use that money as a tip for the people who are doing the hard work...the serving staff, etc.? If you liked the magic and really want to give me something, on your way out tonight, tell the staff or the manager that you really enjoyed the magic and would come back to see me again. Thanks!"
This keeps you on good terms with the staff (who are usually being paid way less than you per hour), makes you appear gracious, considerate, and humble, and also lets the manager know that hiring you was a good thing.
My only problem with declining a tip is that it can be taken as an insult to the person giving the tip. They obviously thought enough of your performance to warrant pulling out his/her wallet and offering you a monetary complement. To refuse it is somewhat insulting. You're saying the restaurant's money is more important than theirs. You may not mean to say that, but that's the way it comes across. The waitstaff can earn their keep just as well. If they do a good job, the person will tip them accordingly. I don't think their tip decreases just because the patron tipped the magician as well.
As far as keeping you on good terms with the staff, why don't you simply keep the tips and then personally greet each of the servers at the end of the night with a portion of your tips in your hand. Thank them for "helping you out" or whatever you want to tell them. That way it's not the patrons that tipped them...it's YOU who tipped them - and they didn't really do anything. I feel that's much stronger than just having the patron tip them (which they are going to do anyway.) The waitstaff is a lot more likely to actually LIKE having you there when they know you are going to split tips with them. They may even ASK the manager to have you there more often.
One other point: while it may be true for a lot of the restaurants....the one I work has waitstaff that makes about 4 times what I get paid by the restaurant. Put it this way...the average check for a dinner for two runs darn near $150. 15% of that (for just normal service) is around $23. These guys probably make closer to 20%. Now multiply that by a minimum of 4 dueces a night, probably a couple 4 tops and the money adds up really quickly. They walk with probably $225/night! Ok, so that's not exactly 4 times what I make, but I think you get the point. At least in my case I'm not worried about the waitstaff not making any money!