You probably should leave a tip.
-Are you on a first name basis with the waitresses? If so, you can’t simply not tip, but you may be able to get away with tipping less than 15%. Or you could tip big one day out of the week, and skip other days. Or, my favorite, you could leave a lottery ticket every now and then (but not all the time) in place of a traditional tip.
-If you are not somewhat friendly with the staff, tipping could get them to turn on you, and put nasty stuff in your drinks.
As tinyfaery mentioned, tipping big will get you better service. If you’re not on a first-name basis with the waitresses now a few big tips will get you there.
If it’s only you eating there the tip can’t be too much. Usually a one-man breakfast meal is less than $8; lunch >$10; dinner >$15. The tip isn’t as back-breaking as if you were with three or four other people.