Digital BW, The Print

Hi all, question for QuadToneRIP Linux users.

Has anyone managed to print an image *exactly* 
centered on a page?

When I specify centering, the image is always 1/4"
further more inset on the left side than the right,
though it is perfectly centered vertically.

I've been trying various options, but none seem to
affect this and I've run out of ideas. Same problem 
occurs on both 8x10 and SuperB, by the way--always a 
quarter inch more white space on the left side.

Hints appreciated,
David