The residue that WD-40 leaves behind is mostly going to act as a corrosion preventer. Maybe not the best, but certainly not the worst.

Also, for the advice given back on the older posts of the Garmin Forums, it's not intended that the WD-40 be sprayed on. That'd get too much in too many places it doesn't need to be. For the times I've done this, I just get some on a Q-tip or a drop on the end of a pointed thing such as a pencil and touch that to the contacts. A little dab a do ya.