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.
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Religion is a good thing for good people and a bad thing for bad people. --H. Richard Niebuhr