We all know that "a stitch in time saves nine" and "an ounce of prevention is worth a pound of cure", but sometimes the opposite is true. A stitch before its time is a waste of time and thread. If you wash your car right before you go off-roading, you're basically throwing soap away.
I've heard the phrase "you ain't gonna need it" in software development circles related to this sort of situation, but I'm looking for a more mainstream idiom, proverb, or cliché, if there is one.
The situation would be someone who is expending a lot of time or resources on something that might in some situations be a good investment, or even seems like a good investment at the time, but ultimately it doesn't pay off.