You get very few rebel launched attacks, and the rebels tend not to build up their provinces, though sometimes there are pleasant surprises when you take over ownership. I will considering bribing them as well, as this helps with not having to rebuild. As to keeping borders to a minimum, if I can, I will do it. But since I did a lot of raiding earlier aganist the HRE and the now the Italians, the Hungarians, and the Polish, leaving behind rebel lands was easier and more cost effective than trying to hold onto a bloated empire with mercenaries.