If the projector is an offset lens, then I'll set it differently than if I need to have the lens at/above the bottom of the screen vs below it.
I find that if the screen isn't taut, an offset lens (like in the NEC M352WS) will reveal wrinkles and not look nice. A "normal" short-throw lens will lessen this, but you still have to deal with barrel distortion from the fisheye.
The first-surface mirror/folded optics type (Panasonic, for example) can lessen this, but you lose out big-time on lumens.
As far as rear vs front projection, I like not having a hotspot with rear; front is fine if you need the viewing angle and can't afford the loss of seating RP requires.
- Jordan Wolf