As the others have mentioned, it shouldn't be an issue. Assuming your scalp is healing nicely, it should be totally safe to do. At 5/6 weeks post-op, the grafts have been secure for quite some time, so I guess the main concern would be not to irritate a potentially sensitive scalp. I'll be interested to see what their explanation is.
I've found that some surgeons simply tend to be overly cautious with these sorts of things. I remember seeing a post on a different forum a couple months ago where the surgeon told the guy he couldn't take clippers to his head for 6 months after the procedure - which, needless to say, is quite extreme.