Is Worpdress 2.8.6 upgrade required ?
Each time there is a new release, you’re invited to upgrade your wordpress setup.
In a perfect world, everyone is doing so. But in our (crazy) world, you may want to defer the upgrade. And regarding wordpress version 2.8.6, you would be right. Provided you’re not concerned by this security warning:
2.8.6 fixes two security problems that can be exploited by registered, logged in users who have posting privileges. If you have untrusted authors on your blog, upgrading to 2.8.6 is recommended.
In other words, if only you (or trusted users) have posting privileges, you may just wait for next wordpress release 
[...] unlike the 2.8.6 release, the 2.9 release is worth considering the upgrade, with new features targetting end-users, [...]