I personally haven't heard anything one way or another. Given that later in the game the AI just simply refuses to interact with you on a diplomatic level (I know it's done to increase the challenge, but it just feels wrong), you're often stuck resorting to marrying from the slightly lesser stock.

The only time I went to any extreme effort to groom great kings (my Byzantine campaign), the mid to late game lack of royal marriages didn't seem to weaken the line (if it was, I would have loved to have seen the uber-heirs I would have produced). My heirs were all coming out with generally good stats and there was a two or three generation run of 'warrior' virtues of various levels during a period where no one would marry my kings.

Then again, despite what seems to be runs of hot and cold kings, there is still little decisive proof that traits or stats or hereditary to begin with...

[This message has been edited by Ii Naomasa (edited 10-14-2002).]