Tell it! I've noticed that in large groups, there's a always a bad apple or three that will chip in only for their own entree, withhold on their drink ("I only had water!") and tax ("uhhh... and what would that be?"), and weasel out of the tip, too, if they can.
My two friends and I get together for dinner every few months and usually take quite a while (talking). Last time we gave 25% for average service (we felt guilty for taking her table for so long) and the waitress was so grateful that she came back to personally thank us for being so generous. Then she said we could sit there for as long as we wanted and talk.