Personally, I wouldn't do it if my gut feeling was that it was about ruling out autistic spectrum disorders, but I would if I thought I might be ruling one in, because I've seen a child with an ASD really benefit from a special communication class and other services to which he wouldn't have had access without such a diagnosis. OTOH, if in your area there's nothing like that (do you know?) then it might be just as good to read up on ASDs and, as you say, take that into account without worrying about a label.
I also think labels (including GT of course) are best thought of as approximate conveniences rather than as Platonic reality. That is, there may be no real "fact of the matter" about whether he has such-and-such syndrome, let alone about which of his behaviours are caused by what. He just is how he is, and your job is to find out how to bring him up best :-)
