I think it's a phrase commonly used in the US, when losses and gains OR advantages and disadvantages cross each other out. E.g. The new diet plan is a wash. It reduces your risk of getting cancer but increases your risk of suffering a stroke. E.g. This notebook has a 1400x900 pixel 14" screen and this other one has a 1280x768 17" screen. I'd say it's a wash.
I think it's sometimes also used to mean "let's call it even" or "let's call it a draw".
I hardly hear it in Singapore. So maybe someone more familiar with this phrase could help with more examples.
