If I see any mention of version (either software version which I lookup on http://packages.ubuntu.com/ or Ubuntu version) I tag it accordingly.
It could be used for determining the age and relevancy of the question. Tags can be used for searching: https://askubuntu.com/questions/tagged/11.04+upgrade
I find the version tag more useful to questions related to upgrades, bugs, drivers and the use of new software like Unity. Software recommendations and shell-scripting questions are examples of questions where the Ubuntu version does not matter.
