It depends.....
15% is the norm here in Toronto for standard service.
Now, here's the catch.....
Say I go for breakfest with my current female friend at Sunset Grill on a Sunday morning and the bill is 20 bucks. 15% is 3 bucks.
Lisa is our waitress and she does everything that Francois did for me the night before at Scaramouche, only at Scaramouche, the bill was $500.00 for the two of us.
Lisa came to the table, she took our order, came back and asked if we wanted anything else, we probably asked later for another orange juice and the bill is 20 bucks.
http://www.sunsetgrill.ca/
In that situtation, I'd probably leave a five.
Now Francois at Scaramouche. The bill for the two of us for dinner is 500 bucks. Francois took our order, came back, asked if wanted anything else, probably got asked for a glass of water, then brought desert.
http://www.scaramoucherestaurant.com/
For his efforts Francois gets 75 bucks from me (maximum) unless he detailed my car while while we were having dinner.
BTW, you tip on the subtotal which does not include the taxes.
Lastly, one other thought, in my travels in the US, I find your average restaurant bill to be cheaper than in the Great White North. (It's cheaper in the US, they don't tax the shit out of you, and the wait staff makes less.) So, it would probably be a good idea to leave a bit more.