You're right that there's a tension here, between wanting to avoid giving away information about your hand (which would call for betting the same amount with your entire betting range) and wanting to exploit your opponent, which usually means doing something that is itself exploitable. If you bet smaller with your bluffs and he plays back more often at small bets (whether he's consciously trying to exploit you or not), then you are getting exploited.

Some patterns are harder to notice than others. Even if people who will tell you that balance doesn't matter in tournaments (these are usually people who don't really understand what balance means) don't vary their preflop raise size based on their hand. That's because they know that opponents can very easily pick up on this tendency.

I think flop, especially heads up with the big blind, spots are similar in that they happen often, everyone has a lot of experience with them, and people tend to play them a little better, which means you should try not to give away too much information with your sizing.

Once you have a more clear idea of your opponent's hand, especially if you think you know how he will play it, then you can worry less about concealing your own hand and just do whatever you can to optimize results against his hand. So if, for instance, you were sure he had a 7 and would never fold it, then you could just shove the river with your boats and check everything else. If you thought he would always fold a 7 (maybe there are flushes and straights possible) to a shove, then you should shove your bluffs but bet smaller for value. If you have no idea what he'll do with a 7, then that's a situation where you'd aim to be balanced yourself.

The thing about the example you give is that a 7 is a very small part of your opponent's range, so although it's good that you're thinking about it, it's too soon to start thinking solely about maximizing value against that one exact hand.

This is great stuff to be thinking about, though, and I can promise you'll continue to wrestle it with it for the rest of your career. This is the meat of poker right here.

You can get exploited without your opponent ever realizing what you are doing or making a conscious effort to “play back” at you. For example, let's suppose that you happen to be seated with the world's biggest nit, and on the river he will fold everything except the nuts to any bet. But you have a rule for yourself that you never bluff in low stakes tourneys unless you have a read that your opponent is a nit. Since you have never played with this guy, you don't know what a nit is, so you never bluff the river against him. As a result, he gets to show down a lot of weak hands that are nonetheless better than yours, and you lose a lot of pots that you could have won. This guy knows nothing about how you play, but you chose to do something exploitable, and you got exploited as a result.

Balance should be your default. If you choose to do something exploitable, it should be either because you have a player-specific read (this guy folds too much, so I will bluff him more often) or because you feel comfortable making assumptions about how he will play based on his appearance (if you're playing live) or just what you know about most people who play at the stakes you are playing. So maybe it's true that most low stakes players really are too loose and you can do better than balanced play by making a rule like “never bluff at low stakes”. You'll still get exploited if you run into the guy above, but you'll also exploit any players who really do call too much on the river, so overall you might be better off using this rule in the absence of a read.

The point is that you should understand when you are doing exploitable things and no what particular leak you are exploiting, so that you are capable of changing your play in situations where that leak does not exist. This is something that holds a lot of lower stakes players back from winning in tougher games, because they aren't really learning to play poker, they're just learning to exploit a few specific mistakes that are common among very weak players, and then when they play with better competition they have no idea what to do.