I can't speak for Carrara, but in the case of Poser 7 you don't have to worry, it won't allocate excess memory just to get to a power of two. In the case of FireFly rendering, you don't have to worry about texture size at all - if you use 1,200x1,200 or 12,000x12,0000 textures will make barely any difference in terms of memory footprint.