From what I understand (and that's not a lot), data from one rom to another sometimes doesn't play well. I think this pertains mainly to system Apps (which I don't restore anyway). Even so, to play it safe, unless I really need the data from a given app (angry birds etc..), I just restore the app. I don't mind having to enter my email address again or whatever I lost by not restoring data. That's just how I do it. I'm sure it's not absolutely necessary. Either is mounting system before installing a rom, but I do it anyway. To be honest, I jump around roms all the time and have not had to sbf once. Not one bootloop. 0 issues with cm7 so far. So, I just keep doing the same thing to keep my sanity
Hope this helps a little
CM74D2
That pretty much covers it. Data does not always play well when transferred from one ROM to another. It can cause issues which would otherwise not be present. As Idopa said, this happens most commonly with system apps, but it has been known to happen with certain user apps as well.
It is also always a good idea to reboot after restoring apps and/or data. Some of the data will not be loaded into the app correctly until a reboot occurs. And as I said earlier, I ended up with issues when I first restored data on CM7, but a wipe of cache and dalvik cache took care of my issues. So if you do end up with issues after restoring apps or data, first reboot, then if it still persists, try wiping cache and dalvick cache.