There is no such thing as a tip too large. I would never tip less than $5.00 for any performer. You will just get more attention if you tip something like $20. Nobody will think you're asking for sex, and even if they thought that's what you wanted -- you're offering a tip, they're not signing a sex contract. No dancer will turn down a tip from a generous man. Motives don't affect her or him. If the client wanted something "extra" for it they're just not going to get it, and that's their problem. It's the dancer's job to make money dancing and if they get tipped for dancing, they take it at face value.
For a stage show, $5 - $10 is reasonable. The more you tip, the closer the performer will dance to you, so it's your choice. After a few good lap dances, anywhere from $10 - $50 is a reasonable tip AFTER the price of the dances, but it's not always necessary, especially if you're spending over $140 or so on dances.
If you keep a dancer at your table for a long time leading her to think that you might get a dance and then don't, it would be nice to tip her for the money she lost spending time chatting with you. $5 - $20 for that, depending on how good a conversationalist she is and how long she stays at your table.
REMEMBER: Most of these performers have to pay to work at the clubs -- the only money they make comes from you.
I have turned down two tips. Once was when an extremely drunk guy tried to tip me $200 and his friend told me that he didn't know what he was doing. The friend tipped me $20 and I didn't take the guy's money -- he was barely conscious, it felt like stealing. The other time was when a woman tried to tip me after having a conversation at her table for a long time, but I she was there doing research for an essay and I could tell she was really nervous. I told her to use the money to buy herself and her friend some drinks and to enjoy their night.