We raise in position preflop. We get it HU against an EP player. EP player checks flop.

Any reason not to c-bet 100% of the time here?

Cuz I probably do and I'm just checking up to make sure that's acceptable.

If it is the same EP, and that EP is paying attention, yes, you should do this less than 100% of the time. Or, if there are other players at the table who pay attention, you should do this less than 100% of the time.

Otherwise, cbet away.

depends who it is - there is the type of guy who limps/calls only with pocket pairs and big ace + broadways. but you probably won't choose him as a light iso target in the first place. but even then, c-bet more often than check.

If you're cbetting 100% that is terrible. You need to adjust to your opponents. If you see opponents check/calling middle pair type hands and you're not going to 3barrel then you shouldn't cbet; if it's a really wet board, you probably shouldn't cbet. If villain's flop c/r% is really high and you flopped a good draw on a wet board, maybe you check it back one time. There are tons of spots where you should not be cbetting and it's all player/situation dependent.

This is so situation dependent

Firstly, stack sizes??!!

If its around 100bb and we are playing supernit tight, (e.g. 1 or 2 hands per hour, no set-mining), there is a style of play whereby I like to wait for AQo+ 100% cbet heads-up and double barrel with AK often. Its just so hard for opponents to take a line which is limp-call/check-call/check-call vs a guy who seemingly has QQ+. It can be very effective but it requires e.g. a 7-hour session and few people have the patience to play this tight. But you'll almost always know where you are in the hand. Get a deck of cards and experiment playing a few 9-handed hands like this and you'll see what I mean.

Alternatively, if its deep-stacked, possibly short-handed, I think I have a read on the table and I'm playing loose (e.g. raising in position after 1 or 2 limpers with hands that are only 'better than average') then I'm going to be checking back a lot of flops even with TPTK - i'm probably cbetting hands with no hope of improving and monsters where there is a chance villain also hit such as bottom or middle set on an A-high or K-high flop. I don't mind loosing a number of small pots or even showing down weak hands. My plan is to get two-pair or better hands and stack someone (who mistakes my preflop looseness as spewyness) or extract thin value on the river where possible. I'm also warey that if we're deep and I'm playing loose and c-betting too much then I can easily be check-raised off of almost any flop by any non-maniac. In this kind of deep-stack situation the flop raise becomes the new cbet - villain knows we hardly ever hit the flop hard enough in order to be able to continue with two streets left to come.

My point is: there is no substitute for looking at the table, looking at the stack sizes and deciding on a game plan (including a cbet frequency) which we think will be most profitable for the situation.