Is it acceptable to ask elementary questions about Linux that will be of interest to Ubuntu users (e.g., how do I lock the screen in Xfce?) though one may be using another distro? Such questions will probably have similar answers for Ubuntu as well.
Seeing as there is now a Unix and Linux StackExchange site (public beta opens on 17th August) I believe such questions should not be encouraged.
XFCE is an ACCEPTABLE question in my opinion - because Xubuntu (a supported flavor) uses XFCE as it's Window Manager. However As was pointed out, Unix and Linux StackExchange is in private beta and will soon be public - those questions will likely be migrated there.