Tipping is very very annoying in the States, but I still do it because I know how poorly compensated they are.
Oppose credits strongly. Adequate wages should be paid and tips, if given, should have a rewarding nature for above average service.
^ This. What on earth's the point of tipping if it's just going to count as part of the employee's regular wage?
If you like your service, give the person a bonus. That's it.
Likewise, I'd also tip the normal amount in the US, considering how atrocious the minimum wage for restaurant employees is. I'm kind of stunned that this system is accepted.