Don't you need to de-authorise certain apps before you remove them from the system, like iTunes etc??
I've never done this. It'll just count towards your 5 PCs which doesn't really mean anything until you try to activate number 6. Then you can just deauthorize all of them.
I guess you could go through all your apps and make sure they're unregistered. It might be important for Adobe stuff.
What about apps from the app store etc? I know some of them you can only install on 3 systems?
What if you have them on 3 already, and wipe one of them to install on another? FCPX for example - how would you deauthorise that, or do you need to?
I honestly can't answer that. FCPX's FAQs state you can install it on Apple computers you own on control. If you have any software that might need to be deauthorized you might want to do that first. I think the only one I have would maybe be MS Office. Although with my Windows versions if anything I'd just need to call in and get it reauthorized.