i am intensely suspicious of any "suggested tip" amount preprinted on any bill.

first, many of them suggest things like 18%, 20%, or even higher. a proper tip is 15%.

second, as dhammer noted, the suggestion may be on the post-tax amount, not the proper pre-tax subtotal.

third, the amount may be entirely wrong! the last time i noticed a suggested tip amount (something like 18%), i calculated what it was 18% of - and it bore no relation to anything on my bill. the number made no sense at all.

fourth, i suspect that restaurant owners put things like this on the receipt to avoid paying servers directly. many states allow restaurants to pay less than minimum wage, but the owner has to make up the difference if the server does not get enough tips. by using these inflated suggestions, the owner runs less of a risk.