Yea, this is a topic I have dealt a lot with as well. Most often when I make "corrections" or additions to a map it is part of the map that is off the edge of the screen and normally not visible during game play unless you look in a name table viewer or something. I will also complete make up graphics using tiles from other parts of the game and some times even adjusting those to make the edges of a map look nice. I did this a lot in the following map and in my opinion it looks great and was worth doing:
http://www.nesmaps.com/maps/SuperC/SuperCMapArea08.htmlMy guess is just looking at the map you can't tell all the stuff I added but there was a lot all through the level.
Also I do it a lot for SNES games that have a BG that scrolls at a different speed to make it line up with the map, or to make it so it repeats. I did this in Donkey Kong Country and Actraiser quite a bit. I will normally leave mistakes in the map from the game if it is in the FG of the main playable area that is highly visible in the game.
Example of a SNES map I modified the BG layer a lot to make it fit with the foreground of the map:
http://www.snesmaps.com/maps/Actraiser/ActraiserMapFillmoreAct1.htmland
http://www.snesmaps.com/maps/DonkeyKongCountry/DonkeyKongCountryLevel13Map.htmlHowever in my Photoshop files I always make a folder called "Fixes" and put them all in there, so I can easily turn them off and see the original map from the game.
So to sum that all up, as long as it does not effect the game play and it makes the map look better or how it "should" have looked then I normally go for it.