It depends on how much stuff you need to move around. The only times I've taken my fish out to rescape is when completely changing the gravel. If I want to put an object where they are swimming I just place it very very slowly so they have time to move out of the way and double check to make sure I don't crush anyone. If you want to totally rescape the tank and move everything around it'd probably be a better idea to take the fish out, especially if there's not many fish in there and they aren't too evasive. If the fish starts to attack you once you start you could take them out.