i would say if you're protected by a router, or not connected to the net in any way otherwise - yes, leave it on. it runs routines as cron jobs that clean up files and such, and maintain your computer usually in the wee hours of the morning.
i've left my powerbook and a g3 running 24/7 for 4 years now, no problems, no hardware failures. it's been all good.
personally i put my powerbook to sleep every night, its just what i prefer, your computer should be fine running all the time, and i know that sleeping it will be fine too, just do whatever you are more comfortable doing