Waitstaff/Servers of Houston, please help settle dispute regarding tipping (kinda H-Town relevant).

So I've never waited tables but that wife lady that stays in my house has (at Bennigan's in the Galleria back in the day if you wanna know).

When I want to recognize a server that's gone above and beyond, I'll tip 30%-ish and give them $10-$20 cash directly. My wife thinks I'm a fool because she says it doesn't matter how I tip the server, they're gonna use the cash to pay the house at the end of their shift.

My thought is that if someone gives me cash under the charge slip and says "that's for you", they're gonna know or segregate it so they don't have to put it in the tip pool or whatever. Also, I think maybe tipping 50% might be a flag in the POS system. I've even asked her "so I should just put it all on the charge slip without the cash?" to which she says "yes". She thinks I'm a moron since I never waited tables.

What's the best way to give a server a better tip so it all goes to them?