Katie,
Actually, Naphtha is similar in composition to mineral spirits as far as flammability & health hazards go so it's not that dangerous to use compared to other solvents. Yes, you are right, it also might damage the card since it doesn't evaporate very fast.

The big problem with Acetone is that it has a very low flash point (0 deg. F) therefore it is very, very combustible, a spark from static electricity can light it up. But, it will evaporate very rapidly & probably not do any damage to the cards.

In my work that I do, I use several different solvents. You can tell who the people are who don't wear protective gloves &/or respirators. The skin on their hands has become all dried out from repeated exposure to the solvents (washes the oils out of your skin). Also, some have respiratory problems, dermatitis, etc...

So occasional use probably won't hurt you. But anymore than that, I would definitely recommend wearing chemical impervious gloves & a respirator when handling any solvents. Below is a link to a site where you can find info on all that nasty stuff.