[BluffTHIS!]

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I don't think I've ever folded the nut flush or nut house fearing a straight flush or quads. Especially at the lower limits.

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The last sentence is the key, lower limits and max buyin games. But with very deep stacks, even though such situations are rare, you should be prepared to fold such hands or at the very least the amounts involved could cause you a losing month. A losing hand is a losing hand no matter how unlikely the nuts may be.

But there is a situation involving nut full house versus quads where you are going to get screwed almost everytime unless your table sense is very keen. If you flop the nut full with like QQ on a flop of Q66 and you slowplay the flop and then raise even just a min raise on the turn, and two players call, then it is easy to think they are fish each with only a 6 or maybe Q6. But if one of them then bets into you on the river or raises you when they have position and a card higher than a Q did not fall, then you are surely beat even though 2 callers on the turn would make you think there is no way either one could have 66. The explanation for your opponents' behaviour is either that 1 of them is a total fish overcalling with an overpair like KK/AA, or that they are colluding in a somewhat sophisticated manner. I've seen both at high stakes. Again at lower stakes online, you are probably just going to payoff everytime assuming you both did not start out with considerably more than the original max buyin.

Regarding the OP's hand, slowplaying a flopped nut flush isn't bad and is often the way to win more than just a pittance in an unraised pot especially. If they have a set they are a dog to fill, and if they have a worse flush they often put you on a set and keep coming. Depending on the player you can even occasionally decide to keep playing soft on the turn, provided that, you don't allow the opponent to underbet such a small amount that he would be pricing himself in correctly to fill. In that case you must raise everytime.