The danger of repairing a microwave is mostly the nature of the power supply, there is no residual radiation risk in an unplugged and discharged microwave oven. The PSU operates at lethal levels (High voltage and amperage) and the capacitor for almost all models bleeds off slowly, so unless you really know what you're doing and you can discharge it completely don't even consider removing the cover unless the thing has been unplugged for a long, long time.

I've taken apart of lot of electronics in my life and I absolutely would not operate on a microwave oven while it was plugged in. Ever. Repairing a microwave is rarely worth it even for a professional, parts and labor are expensive for obvious reasons while new microwaves are fairly cheap.

So, if you can see the problem with the little wheels and wipe it off with a towel, do that. If you feel the need to go for a screwdriver or anything more heavy duty, just go buy a new one. It's not worth it.

Be safe with your microwave!