If you want accurate positioning in Poser, you'll have to start using all your camera and display options. The top, side, and front views are very useful in such tasks, and Poser 5's multi-pane windows allow you to view a scene from multiple angles simultaneously. The wireframe and outline display modes provide much-needed "x-ray vision" when dealing with obscured or crowded scenes. You can also use the Hierarchy Editor to toggle off the visibility of specific actors and figures to better see what's going on.

But yes, Poser isn't really intended for content creation, despite geep's evidence to the contrary. It's meant for use with pre-made content. You can use the included primitives to quickly slap together some simple environments, but if you really want to build stuff I'd recommend a modeling application. Try out some of the freeware that's available (Wings, Blender, etc.) before investing in one of the commercial packages.