Poser 10+ is actually more practical, unless you intend on not taking full advantage of the features that have been introduced in 10 and 11, or intend on trying to support two versions of the same figure which can be a confusing nightmare for potential users. Poser 9 can't read subD surfaces for example, and the unimesh system is pretty rudimentary. Facial rigging is possible in 9 and 10 but only 11 will give you the ability to create control handles for the rig or an additional pallet for its dials. Control handles could be built by hand of course if you want to go that route. and then there is HD morphing which only 11 will read. There are other issues too but I'd have to go back and remember all of them. Of course if it's just the weightmapping you're interested in, V4 has already been weightmapped a few times.

IMO, any new figure that someone were to create at this point should do its best to encourage users to upgrade to the latest version of Poser, while giving consideration to those who haven't upgraded yet. Poser 10 can be found pretty cheap after all, and the longer 11 is out the cheaper 10 will get.