Jeez, a fella goes out of town for a day or two, and lookit all this stuff! OK, here goes:
"Parasites" and "simple decisions" don't belong in the same sentence. #1. Often, you don't know what the animal has, if it has anything at all. #2. An animal may be stressed or sick from a variety of causes -- just sometimes the parasites are the cause, but always they are capitalizing on an animal with reduced physiological defenses. New imports are often in bad shape for many reasons including parasites. #3. The treatments can be hard on the animal, and may put an already stressed new captive over the edge. Vets don't always know what they're doing, either, so be careful.
No one here is making a critical distinction -- between directly transmitted and indirectly transmitted parasites. Parasites that can be directly transmitted are able to increase in number within an infected animal, and to infect other animals in your collection. These are the 'bad guys' that can really cause a world of hurt. Almost all single-celled parasites (amoebas, bacteria, ciliates, flagellates, Cryptosporidium) that are not carried in the blood are directly transmitted, as are tapeworms and many nematodes. Some infective stage is shed by your monitor, and can be easily picked up by cagemates, or work its way throughout your collection on cage and handling equipment, your hands, crickets crawling between cages, etc.
Indirectly-transmitted parasites require either a vector or one or more intermediate hosts (other species in which different life-history stages of the parasite must live before they can infect another monitor). These include blood-borne parasites such as fliarial worms, malaria, hemogregarines, etc., and larger critters such as flukes, pentastomids, and a bunch of less common things. With these, the monitor has all the parasites it's going to have when you acquire it, it won't get any more unless you feed it prey items (like frogs) that are intermediate hosts. This isn't a get-out-of-jail-free card, sadly, because the new import you get might have gotten a huge dose of immature stages while awaiting export, and could arrive with that parasite load just ramping up to high levels.
What to do? Judgment call! If you have a competent vet, it does no harm to take a couple of fecal samples in, maybe 2 weeks apart, to see what the animal might have. A single fecal isn't definitive, unfortunately, because some of these things go in waves. Most vets aren't very good at identifying blood-borne parasites, and of course you'll not see these in a fecal sample, either.
Once you have an idea what the animal might have, only then is it time to consider treatment. Obviously, if the animal has some indirectly-transmitted parasite in low numbers, most people would opt to simply take good care of the monitor and assume that the parasite will clear itself in time. Directly-transmitted parasites are more worrisome -- for example, if your monitor has crypto, there is no effective treatment, and it poses an immediate and very serious threat to all of your other animals. Other protozoans? Hard to say -- I'd advise a cautious approach, wait and watch, while making as sure as you can that you have effective quarantine procedures. Sometimes monitors do seem to shed or at least control these 'bugs', but the danger is that they are always there, getting spread to other cages, and just biding their time until the infected animals happen to have some stress, such as cycling, that sets them back a bit.
People who are serious about having healthy animals always have a strict quarantine period before any new acquisition is put in a position to spread its 'bugs' around. Same thing, if you are at a show or visiting a friend, regard every animal whose history you do not know as *infected* with something that you could bring home. A couple of times a year we have sad stories on these forums, of the form "all my monitors died". I'm not interested in scaring people, but at the same time I see an awful lot of risky behavior described. "Here's Godzilla with my buddy's iguana"; "Just got my new Nile, tossed him in with Pharaoh and they like each other, woot". Most of the time we are just plain lucky, but don't take foolish risks.
