Last time I ate out was $70, I tipped $15; That's just over 20%, I believe.
I always tip; How much I tip on the other hand depends on the restaurant and service. If it's just a cheap $10 a plate $2 soda place, i generally tip 10% or 15%, which usually ends up being 2-4 dollars(performance based ofc). At nice restaurants, however, I tip 15-25% every time.
One time our bill was $35 and I got into an argument with malady about how much to tip, because I wanted to leave a $5 and she wanted to leave $3. It turned into an argument because I took her money (it was her turn to pay for our weekly brunch) and paid with my card so I could tip more, which she thought was petty. I'd agree that does indeed sound petty, but I would not pay less than a 10% tip... Especially since this restaurant is a place we frequent (to the point that most of the waiters know our names, etc).