My vet told me the same thing - he never does preventative deworming for that reason, he only deworms if a fecal float comes up positive - and then he only treats for the specific parasite found. Actually - there is one worm he always treats if any treatment is given because it is often a false negative (I think round worm or something) - but he only treats for it if something else was found. IE one of my kings had two clean fecal floats, no treatment. One had a snake hookworm, he treated for the hookworm and for the other kind of worm.
As far as mites - you don't need a vet to treat them.
Clean the cage, remove water dish, and spray it with "Prevent-a-mite". Let it air for about 15 minutes or so - then put in water dish (which should be new or thoroughly cleaned with a steamer). Spray Reptile Relief on a paper towel and rub the snake down completely but don't get it on his eyes.
I did that with a neonate MBK that had came with mites and a wild caught alligator lizard that was found with both ticks and mites - and it solved the problem.
The prevent-a-mite kills any mites and eggs that may be in the substrate, and the reptile relief kills any mites that are hiding in the reptiles scales.
I now do it as routine on any new aquisition wether I see mites or not.
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3.6 L. getula californiae - 19 eggs (Cal. King)
1.1 L. getula nigrita (MBK)
1.0 Pantherophis guttatus guttatus (Corn)
0.1 Pituophis catenifer catenifer (Pacific gopher)
3.3 Elgaria multicarinata multicarinata (Cal. Alligator Lizard)