I generally go by the rule that if it says you can't camp there then
don't, otherwise it's probably all right. Everyone's heard about the
girl in the Midwest who disappeared in her van from a rest area, but I
think that's more of an isolated incident than something to worry about.
But given the choice I'll look for somewhere off a secondary road than
the interstate.

Last month we were camping off of a scenic overlook drive near a big dam
in central Washington, and the security guard pulls in right at dark. He
was very nice and told us we couldn't camp there, but could at the
company-owned rest area at the bottom of the hill. Not the most restful
night but it was free and there wasn't much public land around.

Cheers,
 Jeff