This is a good question, so thanks for bringing it up. In my experience, every fish I have ever bought developed ich in quarrantine. Every time, I successfully treated it with high temp alone (no salt added). So a few monmths ago, I asked myself this very question, and decided to quarrantine my fish at 87oF (no salt) whether or not they showed signs of ich.
I'm pleased to say that I haven't had a single case of ich in quarrantine since I started doing this - and I've quarrantined at least four separate groups of fish since then. I figure that the elevated temp can't posibly cause any harm, so why no.
John Paul brings up a good point about temperature shock. I float the bag to acclimate new arrivals to the elevated
QT temp. When I transfer the fish from quarrantine, I drain 1/2 the water in the
QT tank, turn off the heater, and gradually fill the
QT tank with water from my show tank above - that way, the fish are slowly acclimated to temperature, pH, and nitrates.