Never thought about Sassafras, but after a quick Google - I see that it's marketed and sold in stores, just like other kinds of smoking woods. Therefore, I'd bet it wouldn't harm you - just not sure about the flavor.
If you want a sweet wood though, and sassafras is available to you for free - maybe try it? Otherwise, pick up a bag of cherry wood chips - or peach...that will give you good results.
Also - go to the "Roll Call" thread on this site, and tell us about what kind of equipment you use, what BBQ'ing experience you have, what you like to cook, how long you've been doing it, etc....it will help others to help you - if we know a little about your background!