If it's a ringed engine then do it on a stand. If it's an ABC and you're not too worried about a complete break in then you could do it in the model (remember, even Jett advise about 40 minutes to run theirs in). One advantage of doing it in a new model is that it gives a chance to find any bits that might vibrate loose

As far as cooling air goes, with maybe a 100mph prop blast then a 60mph airspeed won't gain you much