I've used rest stops along I-5 (Left Coast) for naps during the day on
many occasions with no problems or hassle.  At night I feel more
comfortable at a truck stop.  In Washington State there are 24 hour Free
Coffee stands set up at nearly all rest stops and I'd feel comfortable
at night if parked next to the stand, like some one watching for me.
I've avoided small, remote rest areas with little or no traffic at
night, even for a quick pit stop. In the past (late '70's) I've even
stopped at rest stops and thrown a sleeping bag on the ground to get
some sleep, but that was with a group of about  six guys returning from
a pistol competition.  For some reason we felt very secure :)