Recently a parent at my child's school talked to me about making her will--or rather, not making it. She knows she ought to, but she and her husband got stuck when it came to naming a guardian for their two kids. "His family is better with money," she told me, "but mine is more fun!"
A lot of parents can't get over this stumbling block, and as a result they don't have wills. And if something did happen to them and their kids needed a guardian, a judge would have no clue about who they would have wanted to take on the job.
The best advice I can give is to just name somebody--you can always change it later. But don't leave such an important decision up to a judge who doesn't know a thing about you or your family. Attorney (and mom) Liza Hanks has a great discussion of how to overcome common problems in her book The Busy Family's Guide to Estate Planning.