Sub wont technically unload. The short answer is: it isnt a huge deal. The technical answer is, the placement of the sub along the baffle can have an effect on the response curve. Furthers away from the port as others have noted, normally gives the smoothest response and squeezes the tiny bit of extra extension out. However, there are times where you wont want it furthest away - it just depends on what youre trying to do.
For most setups and the few inches it varies, the difference would be inaudible..