Probably there is no such thing as a "common" TL. Some sites will lag behind because they like their stable running environment, at some (few) sites they will install every new TL/SP at the moment when it becomes available and most others will be somewhere in between....
As long as you don't experience any problems or lack new features because of the oslevel and as long as you can open a PMR without being forced by IBM to upgrade you should be fine. To find out whether IBM thinks that you are using an acceptable (i.e. supported) combination of hard and software you could check with the IBM - Fix Level Recommendation Tool