Unless the trainer you have in mind is one where the resistance unit contacts the rim, rather than the tire, you will want to use a slick. Knobby tires make a racket on a trainer and, I'm told, they are hard on the trainer's bearings. Just get a cheap slick, no sense in wearing out a pricey one on a trainer. And, of course, you will only need one on the rear tire, unless you are riding rollers.