I really doubt she gave you that change just to force you to tip more. She probably just wanted to get rid of the heavy coins she was carrying around!
You're probably right Kaitlyn, but that's the same reason I wanted to tip her $1.50 - I wanted a nice, light $5 bill back, that fits in my wallet, instead of a bunch of noisy, heavy change jingling around in my pocket. The bill would stay with me until I can spend it. The change gets dumped as quickly as possible, because it's annoying.
I'm more miffed about having a bunch of change ditched on me unnecessarily than I am about being forced into tipping an extra $0.50. I'm not that
Kombat, another suggestion, you can always ask for the specific amount back that you would like. In the above scenario, you could have handed her the $20 and told her that you only needed back a $5 bill, the rest was hers.
Regarding auto-gratuity, if it's on there, I don't add any more. Left to my own (and having waited tables to put myself through college), I am an overly generous tipper.