Its generally good practice to have the developer, server and users all running the same version of QV. If you are developing in QV12 and the users are using QV11.20, there may be model elements that work on your version, but not on the users' version(s). So you will be able to share the documents on the server, but they may not work as well as you like (for the users).
It all depends. Aside from the best practices Jonathan already siuggested (don't mix versions, keep everything at the same major release, even patch level), there are quite a lot of bugs fixed between say 11.20SR4 or whatever you are running at the moment and 12.00SR1. Some of these may impact development, AccessPoint behavior, connections and various other aspects but mostly in only minor ways.
One tip: maybe you better wait for the release of 12.00SR2 which should be imminent AFAIK.