It really depends on what you are trying to achieve by going to see the vet.
I always recommend a physical when you get an animal (this is a bigger deal if you are a novice herper or not.) There is certainly value in having the reptile assessed for health, especially if you invested any sort of money into it. Discussing specific husbandry is big. This of course is only as beneficial as the vet is versed in your particular species and you are not.
Sometimes a general physical exam is a good time to decide whether your vet knows what they are talking about. Better then than when your animal is sick and really needs some expertise. Also, once you are a client, it is easier to get in to see the vet since you are one of their clients.
How often? I agree with Cham_man. After the initial visit, besides fecal checks, unless there is a problem, a visual physical is often what is done. If you have the money to do it, I recommend bloodwork every couple years to have a baseline and to confirm various electrolytes.
Fecals- unless you done know where your insects are coming from, (wild caught) fecals may not be necessary.
Sometimes a knowlegable herper knows more about a particular species than the vet. (I don't pretend to know everything. But I do know more than the average herper...) I do defer to some other specialists (zoo keepers, ARAV members, professors, and other herpers)
If you have the time, call all the vets that see reptiles. See if they own any themselves. What experience do they have (zoos, special training, continuing education). Vet schools don't require much exotics to pass, and you can conceivably graduate without ever putting hands on them at all. Those with experience often have gone out of their way to study on their own, or are enthusiest in their own right.
The money that I make on exotics does not at all compensate me for the time I spend with my exotic clients. I do it because it is fun and interesting. Unfortuanately if I did things for free or less, my bookings would be full of people looking for treatment for free (especially frustrating when most ailments occur because they never bothered to research basic husbandry before they bought the animal, or just took the pet store's advice without question). Certainly there are vets that are looking for a quick buck. Keep this in mind though--that same vet would have made 3-4 times more by filling that appointment with a dog or a cat.
Take that for what its worth. Know why your going in. Call and confirm that you will be getting that service before you go in. Do your own research. These forums give a great reference point. A lot of issues are easily solved. When a problem that requires a vet comes up, they are usually forthright in suggesting a trip to the vet.
Good luck,
Matthew