So long as it doesn't get in the way of your hand, no problem at all - probably less annoying than on top.
There is something else to consider when mounting a torch to a gun and that is what the enemy see - if it's dark so that all they can see is your light, not you behind it, then naturally when they aim, unless they have time to think about what they are doing carefully, they will shoot at the light. If you are right handed and your light is on the right hand side of your gun, it is at the edge of your silhouette. If it is underneath, in line, it is closer to the centre of your silhouette and on the left is just wrong, since that is bang in the middle of you.