Do I need to care about mounting hole contact oxidation?
Eventually oxygen will migrate everywhere, having the terminals mated together will stop this process, but if there is a micro gap the oxygen is much smaller and will eventually diffuse into the gap. 

Get an ENIG coating instead of bare copper

Or use deoxit and clean it periodically  

The real question is: is it going to affect your signal? resistance will but the worst thing is thermal noise from the junction. If you're not concerned about turning your junction into a thermocouple and only about resistance its probably not going to be a big deal, just size your contact bigger.