$10 NLHE Full Ring: Cutoff flatting vs button range?

I read an interesting paragraph in a book saying you should never flat button vs cutoff open you should almost always 3 bet or fold "except for balance against a good reg". The reasoning was that 3 betting gives you more ways to win the pot and the cutoff will be folding a lot since his range is so wide, and you also seize the initiative. but would it not be more profitable to flat hands like KJs which have good equity vs cutoff range and try to extract value (or outplay) postflop? please note the info from the book was describing an unknown or standard player at 10/25nl.

Not many players will react to your 3-bet by 4-betting. Most will just call or fold. I think this is some interesting advice. I believe 3-betting from the Button vs. the CO is good with a wider range than most positions. However, I don't believe that you should do this every time you decide to enter a pot.

Some hands you'll want the blinds to come in as well. You might run into a tight CO opener where 3-betting will be a mistake. With all things in poker it depends.

I could be wrong and in fact I'll try this out and see how it works.

it's a pretty good general rule imo, but there are plenty of situations where flatting is still clearly preferable, at least exploitively.

consider if you were facing a super nitty CO opener on the button with 33 for 4% of your chips. if the CO is opening really tight your implieds for stacking him with a small pair when you spike a set are high since he'll have so many big pairs in his range. also, the chance you'll be 4-bet if you raise pre increases, which is a disaster since it denies you the good price you're getting on set mining. so in this case, calling behind with a small pocket pair is clearly better.

another example where flatting is likely preferable is if you have an opponent with a high fold to 3-bet %, and you're fairly shallow with AA or KK. (by fairly shallow i mean if 2 pot-sized bets or less would put someone all-in). here, with a hand like AA/KK and position, it's going to be easy to just shove over top of your opponents flop or turn bet and get action from wider ranges that may have just folded to a 3-bet pre.
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try to think of other situations where flatting would be preferable to 3-betting. it's a good exercise to improve your thinking/game. and then also ask yourself, why is 3-betting a CO opener generally preferable vs flatting?