All of my production code uses 5.6.2, any experimentation or "fun" stuff is usually on the latest stable release, so currently it would be 5.8.6.

A couple of things I think really worth pointing out are that probably early 5.8.x release should be avoided, IMO 5.8.4 is the first one really production worthy, and that before migrating from 5.6 to 5.8 its well worth doing a speed/feature analysis. Some code appears to run much faster in 5.6 than it does in 5.8 and if you dont need the added features of 5.8 it may be that migrating results in slower code. I do recommend making sure you are using an up to date 5.6 release.

A last comment, ive observed that there are definite differences in behaviour of the different versions on different platforms. Users should always specifically investigate ther own circumstances before migrating as good advice from others may be inappropriate in an alternate enviornement.