Ideally what you would like to do (if you can) is to put the PJ on a cart, send it the highest frequency picture you will use (highest resolution and refresh), set the H Width to 90% or so, and with just the Green on see what the projector throw distance is in order to fill your screen. Once you know that, then you can translate that to your ceiling position. This will give you the ideal position. This all assumes that at 90% H width you are getting a reasonable use of the width or your tuges and that someone hasn't screwed with the HWidth inside the PJ.
The 5% forward isn't that important and certainly won't not doing that won't make your tubes wear prematurely. You have to make the raster really small to get premature wear. In fact the light output between the center of the tube and the extreme edges rolls off very fast as you approach those edges, an for that reason you will see that blend and multiple display projectors don't even come close to the edge of the tube.
I for one am not a big subscriber to the "put it about 5% closer" ideology.
AVS Marketplace or Videogon are the places to see F/S ads