The Tip?

So I have a question. Say your hairdresser owns his own salon. Say the haircut costs 50 NIS. Now, no one washes my hair and no one else works in his salon. How much am I supposed to tip the dude? My wife, who "does not believe in tipping, rather believes in overtipping" (to quote Steve Martin), says I need to leave 20NIS. That seems excessive don't you think. I mean, it's his business. He can charge what he wants. The tip, I always thought was to ingratiate you with the various hair washers and sweeper uppers so they did not burn your scalp with boiling water. Do I need to tip him at all?

I had a haircut today. I tipped the dude 20 NIS.