It is really about consistency. If you are dialing in a recipe and want to make this beer over and over the. I would adjust the water. However, I would not adjust in the fermenter, the best pace to make the adjustment would be at the beginning of the boil. You need to know your system, your boil off rate and such so that you have a clear understanding of what you want your pre boil volume to be. If you wait to top off I. The fermenter, or any time post boil you are diluting the hops as well.
If you are just brewing a batch to brew a batch and might never brew this one again, or just want a little extra punch in the ABV then let it ride and just takes some notes in case you find it to be fantastic.