When I have an "imposed gratuity", er a service charge, I will make a point to circle the line on the sales slip where the imposed gratuity is itemized. I then either write in a zero or put a dash across the line where it asks for an additional tip.

If I receive absolutely exceptional service, I might add an additional tip, but in most cases I make a point to circle the line with the imposed gratuity on the sales slip and leave nothing additional.

When it comes to tipping, I tip based on the service. I start at 15% and then go up or down based on the service. I've been known to leave no tip or a couple of pennies for crap service, but I've tipped as high as 40% for absolutely fantastic service.

If I'm familiar with a specific states wage laws, I will sometimes take that into account when tipping.

I also hate the tip jars that are popping up everywhere that never used to exist. It's often setup in a way where they want you to tip your change or toss a $1 bill in before you even receive what you ordered since most of these tip jars are at the cash register.

I will not leave a tip in one of these tip jars unless I am surprised with above and beyond service.