It's kind of like vaccines... Yes, there's a very small risk you'll have a bad reaction to one (I'm actually part of that very small percentage; had to get CDC clearance for my medical exemptions to attend college/grad school... It seriously would've been simpler to pretend I had a religious reason!) but the risk of not getting them is much higher.

I agree with not treating for most things when they aren't present... I know some people who automatically treat all new snakes for mites regardless. I'm not into that (one of the reasons I prefer a completely separate QT room). But when you have a known exposure of something as aggressive as mites it really is worth going ahead and treating. Even wearing the same clothes between holding the snake with mites and the others can spread them.

If you're wrong and they have spread but you wait until you see them, the treatment will probably take a lot longer, and IMO that would be much more dangerous than the lighter 'preventative' treatment.