Just thought I'd throw in my $0.02 as a vet assistant. Inspecting the poop for worms on a regular basis is good, but ultimately useless since adult worms almost never pass in the feces. Doing a microscopic exam for parasite eggs is the best way to determine whether or not there is any evidence of worms in the poop. And, as someone else has already mentioned, one negative fecal does not necessarily mean that an animal doesn't have worms. Intestinal parasites shed eggs intermittently, so a single negative fecal could just mean that the worms weren't laying eggs that day. It is generally recommended to check three fecals several days to a week apart to rule out parasites. For those who are interested in doing their own (not a bad idea and save you some money at the vet) it is quite simple. You need a microscope (most fecals are read on 10x, although I do read most of mine on 20x because it's easier to spot coccidia on the higher power), fecal float solution, and microscope slides and cover slips. There are lots of websites out there with instructions. This one's pretty good: http://www.ableoaks.com/books/fecals.html
Ultimately, whether to worm or not is a personal choice. Personally, I don't see much benefit to doing it if there is no evidence of a problem, but I also don't think that there's any harm in prophalactically deworming either. My plan is to check fecals on my birds every so often and worm if there is a problem. But I do prophalactically worm my dogs each month when they get their heartworm prevention and it hasn't harmed their health in any way.
Ultimately, whether to worm or not is a personal choice. Personally, I don't see much benefit to doing it if there is no evidence of a problem, but I also don't think that there's any harm in prophalactically deworming either. My plan is to check fecals on my birds every so often and worm if there is a problem. But I do prophalactically worm my dogs each month when they get their heartworm prevention and it hasn't harmed their health in any way.