I went from front to rear port in one of my designs.
It's not at all uncommon for some midrange energy to emerge from the port. If the woofer is active up to frequencies where the port is 1/4 wavelength long, you stand a good chance of getting midrange port resonances (imagine talking through a cardboard tube). If that happens, better to aim it backwards than forwards.
As you correctly suspect, proximity to room boundaries will reinforce a rear-firing port's output. This is usually detrimental, and can be a cause of "boominess". On the other hand if the enclosure were tuned with the expectation of boundary reinforcement of the port's output, the net result would likely be deeper than normal bass extension. Audio Note uses this technique with their rear-ported AN/E, which is intended to be placed in a corner. I borrowed the idea and incorporate a variable-length port system in my rear ported designs.
Now if a rear-facing port is close enough to the back wall, the wall will increase the effective port length enough to change the box tuning. This may or may not be beneficial in a given situation. Keeping the port two or more port diameters distance away from the wall reduces this effect to negligible (which again may or may not be beneficial).
As long as the path length around the box from the rear-facing port to the front-firing woofer is less than 1/4 wavelength at the port's tuning frequency, there should be negligible loss of bass impact from the path length difference. In fact, placing the port in a different plane from the woofer would help to spread out their room interaction effects by giving them different path lengths to the room boundaries. In theory this would smooth the bass a little bit.
All that being said, port location is probably unlikely to be a major factor in the overall sound quality of a loudspeaker system.
Duke
