I mount my LR weapons to the rear, while you don't want guys jumping out into cross fire, it doesn't work well on the table. If you can only get out one side, you will not be able to legally disembark more than five models from one of the side doors with the sponsons up front. However, I play BA and don't like assault termies (in favor of Death co). So I have more models inside than other chapters.
As far as the MM goes, any of the top hatches works fine. If you are using the half-model gunner sticking out the top, he can be hard to fit and swivel around in the front slot. However, I use the closed hatched MM mount option which allows me to swivel at the front easier. The top also has much better LoS (full 360) as apposed to the front which would have a limited arc. So there are plus and minuses.
Moral of the story? Magnetize everything and save yourself some headache. This way, you can move the positions around as you please.