Ehh, tip whatever you want. Most waitresses, especially if they are bad to begin with, will just assume you are being cheap if you tip less than 15-18%. I have never once heard one say "I only got 10%, I think I will have to be more attentive next time!". They don't take it as a notice that they did a poor job. The good ones who are having an off day will usually be very gracious and apologetic, trying to remedy every mistake and will know they messed up before the tip amount comes.
Even though I find most of them to be whiny, overpaid, lazy, ungrateful and greedy (the ones I know I mean, I usually can't tell this from being waited on by one, haha), I still usually tip 15-20% because the tips don't just go to them, they go to support staff and bartenders, etc.