You might have already tackled this and it's something you're going to post later on, but I've found that using non-square (i.e., tile width and tile height are not equal) tiles makes the mortar width all screwed up because that's controlled by one value whereas the tile size has the two values. Result being that if, say, the tile width is twice the height, then the horizontal mortar line is half the size of the vertical mortar lines. Which, for most tiling situations, is not ideal.

 

EDIT: Actually... disregard that! I've just tried it out in PP2010 and it works okay, with consistent horizontal and vertical lines. It never used to work way back in... whenever it was I last tried it and gave up because I couldn't work out how to even out the mortar width. Poser 6, perhaps? Anyway... just shows I should check what the latest behaviour is rather than relying on outdated data!