That said, I never shut down my PB. I haul it around everywhere in sleep mode, and I'm pretty sure that putting the HD to sleep is going to park the heads just the same as if you power it down completely. If it's heat you're worried about, don't. It doesn't generate any appreciable heat when sleeping.
Now, personally, if I've been running the PB hot, so that the fan is on, I'll let it sit idle for a while to cool down before sleeping it. I do get kind of concerned that there will be a problem if I close the hot PB (which shuts off the fan automatically) and then put it straight into a nice insulating case to keep the heat in. I'd be concerned about this if I was shutting down too, for that matter.
Of course I won't assume responsibility if your PowerBook crawls out of the case and kills you or something because you put it to sleep instead of shutting it down. If you're the least bit worried (and, really, why else would you have asked), AlphaTech's advice is probably the better way to go.