I know that the industry has more prevention measures but I still feel most tips are under reported to the IRS.
We usually tip at least 20% and always in cash, that way the waiter can report it or not report it to the IRS at the end of the year, their choice.
I've been out several places that if you have six or more people in your party, they automatically add 20% tip to your bill, only they don't usually mention it when they leave the bill, so you have to scrutinize the bill prior to payment, or leaving an additional cash tip would be an extra windfall to the waitstaff.
I've also heard that at some places that each tip the waitstaff gets, its mandatory that a certain percentage of each tip is to go to the busy boys and bartenders. If the service is poor and no tip is left as a result, the waitstaff still owes the busy boys and bartender their percentage, karma's a b!tch!
Went out on a half day headboat fishing trip one time, and it was extremely rough seas, 30 minutes into a 4 hour fishing trip and almost everyone was sea sick, terrible day, when the captain announced on the loud speaker to reel in, we were heading back, everyone cheered that this ordeal would finally be over soon. While heading back, the captain announced that the two "mates" on board that baited your hook and removed any fish you caught, worked for tips only, so be generous...or he'd turn the boat back around and we'd go back out! That was funny!