Well, I ride on gravel paths all the time with skinny slicks. It's really fine.

but I would have a knobby back, slick front. I sort of do that on my mtb. The rear tire loses traction when riding uphill, so i have more tread back there to prevent it. The front isn't as big of a deal, so it's got a less knobby tire on it. I try to get as minimally knobby as possible, just because they do roll faster, and I'm not doing a great deal of hardcore singletrack. Well...at least nothing my bike can't handle.