always?/never? no easy answers for poker---but---there are several considerations if you are looking for ways in which to make a decision easier if a 'brick' comes.
If you are on a mid/low 3 flush, and a couple are dead I'd get out. If I had an overcard or two I might pick one off, esp. with an ace. If the pot is large, you can call even with the mid flush. Sounds simple, but take a sec and ask yourself---how live are my cards? If I make a high pair will it likely be good? Has there been enough action to go for a semi-longshot? Am I looking at hands that are looking like better draws than the one I might pick up?