Yes this is allowed just can't handle the ball in the scrum, like Mr Back did a while ago.
It is a good way of keeping your shoulders square, so that you don't turn slightly into the scrum and hence slow your get away at the end of the scrum. However you contribute a little less to the scrum this way. To help keep you Props backside square binding on to their shorts with your outside arm, will lead to you being slightly angled in pushing their backside towards the hooker a bit, this is best if the scrum is under pressure and you are not worried about opposition back row
D-Quins