OP: implicit in some of the replies above is ... you are not restricted to an either/or (full-on knobblies or true slicks). There are many 'touring' tires with tread available that will give you a nice middle ground. Some other examples would be Schwalbe Marathon Extreme (true mix of road/off-road) or Dureme (mostly road with some lightish off-road), or 1.75 Panaracer Pasela TGs (for the latter balance). Any of these three (there are many others) would be much less work and much better (and safer) handling on-road, and offer much better puncture resistance than most 'mtb' tires, while still offering the benefits of a 'wide', lower-pressure tire. Knobblies will certainly 'work', but unless you were planning to tour off-road only, I can't see why you wouldn't want to change to something more suited to mixed terrain.