Is it rude to just get coffee at a restaurant?

I'm thinking mid-day. I don't always want to have to go to a Starbucks, and I love sitting outside somewhere just sipping on a coffee.

At what point do you think it wouldn't be a problem? Maybe if I left a a few bucks for a tip?

Or in which ways do you see it being a problem. Anyone in the restaurant business want to chime in?

P.S. I'm not talking about an Applebee's or anything, I live in San Francisco and some streets here are just packed with Restaurants.