Aw, Jolteons are my favorite eevielutions! I have a small batallion of them.

(Is that a totally okay thing to do, and I'm just being ridiculously overly sensitive?

No, you're not overly sensitive. With high-level gyms, the resources required to level up and open a slot are significant, and so is your interest.

That said, for the same reason, I don't think it's a particular problem with low-level gyms (say, Level 1-3 at a public transit station); I have had those swiped and swiped them myself many times.