Well some engines need it as they simply will not turn over otherwise (too tight).
Others u can pull start easily - in a RTR for example.
Therefore you don't know just how tight it will be and how bad it will scrape the bore or if you will stress out the conrod heaps.
So I'd say always preheat a new motor to be safe.... can't hurt it by heating when unnecessary!
I've had some that were so tight (and you don't know till you try) that I had to put heaps of oil in and preheat then run on the starter box with no plug in, cos preheat plus oil and it wtill would not turn over with a plug in. Now I just figure I will always spin them up hot and slippery with no plug for a min or so before I even try to start it
