From Dave Grohl himself. In next week's NME:

on tour

I look forward to hearing the anecdote. I've heard a variation of this before from Cobain's daughter's nanny in that god-awful Everett True book. Apparently Kurt really loved PJ. It took him a lot to work up the courage to ask her to come on tour with them. She politely declined, but her declining such an offer made him respect her even more.

Dave Grohl's a cool guy. I've heard about this before but I'm interested in what he has to say about it.