I can't recall if it was here or over at .com, but someone posted that they played a whole game where they married off their princesses strictly to their princes. Their opinion was it had no increased effect on the offspring, at least no more than the usual 'odd number of toes' that crop up naturally. The downside is your princes/rulers will all get secret incest, and if exposed is -5 piety. The upside is all your princes marry young and good probability when he ascends he'll already have some kids in the pipe.

Also, from a discussion long ago with some dev input, foreign princesses do not impact your heirs. A princess from a high command ruler will not pop out high command sons vis-a-vis her father's bloodline. The dev's asked if this is what people wanted in the upcoming patch, but the consensus opinion was no.