To reply from the veterinary point of view, they can only see what they see in an exam. If the animal for some reason stops sneezing during the vet trip then the DVM's perception is that it may not be that big a deal. What bloodwork does is continue the exam to what the vet cannot see by taking a look at how the immune system, and other organ systems are working. I agree that diagnostics beyond an exam are more relevant when symptoms are greater (like a discharge) there is never a problem in doing so.
If your lizard turns out to be healthy, besides the weight off of your shoulders, the benefits are still great.
The problem with many reptile species is that normal blood perameters or values are unknown, and/or are extrapolated from other species. There is a significant difference in the amount of data collected on humans, cats, dogs, and reptiles. And again, with some 6500 species to consider, there is lots of variation withen the reptile feild. Blood work done on a healthy animal begins a personal database for that lizard. From that point on, if the lizard looks even better, or declines in health, the values can be watched as they rise or fall, and thus an interpretation of the data is based on your specific lizard, and not anecdotal evidence from someone else's captives, that may not even be of the same species.