Any tip is better than no tip at all, which is what they get from many people.
I give a better tip when they're genuinely friendly, check back with us frequently and quickly solve any minor problems.
If the place's service is too slow, they probably won't get a tip at all.
At places where I order and pick it up at the counter I might toss a little change in their jar if they're friendly, give a smile and keep things moving along, but I feel no obligation to tip.
As far as I'm concerned tips are not an obligation on my part. Tips need to be deserved.