You don't typically need a rear end alignment when you are doing a front end alighment. You would typically do a four wheel alignment (called a thrust alignment) when you notice uneven wear on the rears.
Actually on many cars (not sure about the MDX) the rear wheels don't have an alighment ability. Someone may be able to validate this.
However, if the full four wheel alignment is only a couple bucks more than the front end alighment, it's sometimes nice to have just to see what the current spec is.
Lastly, you should get a printout of the wheel spec alighment from both before and after the work is done. This will tell you how far out of alighment you were, as well as validate that the work was done properly.
'04 BMW 525
'92 Porsche 968 Cab