I use centimeters. It doesn't appear to matter anyway because Poser is its own beast. I set up a cube in Poser with a 50 mm camera set to x-units away and rendered at x-resolution. I exported that same cube.obj into Vue 5 Infinite and Maya respectively using the same settings. Vue and Maya agreed, Poser was off doing it's own thing, making compositing a little more tricky.

I know there's tutorials about Poser scale and all that, but that seems like the long way around an easy problem.