The biggest reason to go to 6.6.5 is because of the new CMS 1500 form. If you do not need that, look at the other changes from 6.3.3 to 6.6 and see if there is something of value for you (best way to do this is to to the upgrade site and look at the bottom for updated things. It has more a lot of small things in prescribing, but I skipped over 6.3, so I don't remember what was there.
The version in June ?6.7? will have ICD-10 ability (you can switch back and forth to try it) and will be MU certified if you are planning on certifying.
THIS is probably the next big one and probably worth upgrading. That will give you time to bake ICD-10 practices in your office. It will automatically switch to using ICD-10 on 10/1/14, but can be toggled back for any reason (back billing).
