I think the most important thing to remember if you do drive is that your reaction times are probably slower than they should be. I've always kept my distance from other cars anyway, but it's so important to now. Even if I feel great, I still keep it in mind, just in case. I drive about once or twice a week and usually anything under 15 mins doesn't cause PEM, but driving further can cause it. Holding my arms up can be difficult. Usually if I'm feeling bad, I haven't planned to go out anyway, but I wouldn't be able to drive. I rely on my fiancé a lot to drive me places when not well.
I also struggle more driving at night, as it's hard for my eyes to adjust between lit up areas/oncoming headlights and then darkness. Harder to read signs too, so I try to keep to driving well-known areas if I have to drive at night.
Another thing I keep in mind is that anytime you feel bad, just pull over and take a break for a few mins. If you're late to where you need to be, so what, it's better late than having a crash.