Your dealer is correct.
When level, the lenses throw the image up at 12 degrees. What we're interested in is where the centre of the projected image falls.
For a 16x9 6ft wide screen, bottom edge 15" from floor level the centre of the screen is about 37" from floor level. Keeping the projector level, the lowest it could go is about 11" off floor level.
For a 4x3 screen image the centre is 41" from floor level, the projector would be 15" from floor level.
Tilting a maximum of 6 degrees over a 108" throw lowers the projector by about 11" in theory. (the figures change slightly as the throw distance alters as the projection angle increases). 6 degrees doesn't sound like much, but its enough to get the back of the projector touching the floor if yours is a 16x9 screen.
Although this amount of tilt may not look aesthetically pleasing, as long as the lenses are shimmed with 18 degree spacers (12+6 degrees) and focussed correctly then the projector will run quite happily.
If it is the amount of tilt that bothers you, then you could go for less tilt (3 degrees) and sitting a little higher.
Regards