In general, the very front and back cabins on each deck are considered less desirable than the whole rest of the deck, because they're more prone to feeling the up-and-down and side-to-side motion of the ship as it cruises through the water.
That being said, on the newer, larger ships such as Mariner, there is less motion in those rooms than on smaller ships, but it's still there.
Booking one of those cabins would depend on whether there is anything else available for the date you want to travel, and how prone to motion sensitivity you are. If you do decide to go with one of the front cabins, it's thought that the lower down on the ship you are, the less motion you feel. Choosing between Decks 7 and 9 probably won't make a huge difference.
Are there other cabin categories available when you want to travel? Perhaps spending a little more to move more midships is a good idea.