Editing WoW:UI FAQ/AddOns (section)

=== What's the new toc number? ===
Unless you are a mod author, ''it does not matter''. 
* Don’t go in and manually edit your toc numbers
* Don’t use a toc auto-updater program! 
There is simply no need. Use the "Load Outdated AddOns" check box, located under the AddOns button on your Character Select screen instead. Check in at the location where you got your mod(s) from originally over a few days after a patch, to get the properly updated version from the actual mod author.

By going in and changing all the toc numbers yourself, you are completely defeating the entire purpose of them in the first place.

If you use "Load Outdated AddOns", without making any changes to the toc numbers (i.e., the proper way to deal with patch days), then when something stops working, you've got some hope of figuring out which mod(s) aren't working right, since you can go back to the AddOn button on the Character Select screen and see which AddOns aren't updated yet.

If, however, you go in and manually change all the toc numbers, so far as WoW is concerned, they are all "current". Now what do you do when they stop working? When all of a sudden you've got error messages popping up all over the place? How do you know which ones are actually current and which ones are outdated and probably the problem? You've no way to tell any more. ''Leave your toc numbers alone.''