It depends on the game and the characters you are playing, as well as the setting (offline casuals, online, tournament or money matches, warm ups vs no warm ups in those settings etc.)
A "baseline" damage drop of 3-4% is ok when you are picking up a character you really want to play. However, when practicing with those characters you should practice the harder stuff every now and then, just for shits and giggles. Don't beat yourself up if you can't land them. One day you go on practice mode and you just magically land the max damage stuff and you finally get the timing. Now you have choice, depending on your general preparedness to play at that time and the setting you are playing in, you can pre-determine what you are going to do. Maybe go for the ez stuff if you are not really feeling yourself, maybe try the harder stuff, but revert back to the basic bnb's if you drop a harder combo... or just try to land the harder combos anyway if you don't care about the outcome of the match.
For tournament play combos there are variables to consider. You might be listed for 2 different games, even if not you might have to jump into a game with cold hands. The screen you use at home might have lag but the tournament organizers got lagless monitors or the other way around, if you rely on a visual cue more (usually the case) instead of an audio cue to time your combos properly then this can mess up your timing. If you haven't had a chance to play casuals at the venue before your tournament matches then even the ez-mode bnb's can take some time getting used to.