If you had your CAT 5 installations certified then you should have no problems with CAT 6 as long as you are a careful installer.
If you are asking about certification, IMO it is just another way for those IT types to make sure they are getting their money's worth and hold you responsible. If you do a careful job it's not going to make a bit of difference whether it's certified or not to their computers but chances are certification testing will force you to go back and replace some runs or reterminate some jacks.
If you want to spend 5 or 6K you can get your own test gear and do it yourself (no such thing as being "certified" to do this) but having it done by an independent company keeps honest people honest and is better than you spending money on equipment that will be an expensive door stop in a few years when CAT 7 is introduced.
If you haven't got the message by now I stay far away from those jobs that want their wiring certified. It's not that we are concerned about our work but we just don't need the aggravation, especially for only 25 locations.