It's only important if you like keeping the baffles intact on your boat... or want to push a flotilla. Transferring the thrust of your prop through your frame and on to other boats frame to frame (oarlock to oarlock on your sides and diagonal from corners of frames if you stack, wedges and diamonds are the most efficient shapes - always push, never drag...) is always the most efficient method, both for handling and speed. There's a reason almost all the old rubber motor mounts on old Avons and Campways/Rikens split - UV partially responsible, but also just not a good design for transferring that torquey power to the boat. Your transom doesn't need to be against your back tube, by the way - you can leave a little space there (inches, and the less the better) and it will help with turning if you're running a short(er) shaft where the powerhead is too low to clear your tube.
No comments on short shafts...