I haven’t taken any SAT subject tests yet, but I plan to take the Math 2 and Biology test. I know NYU doesn’t require subject tests but their common data set says they consider them if submitted. Should I wait until I take the tests then apply regular decision, or just take the ED boost?
Depends on the rest of your test scores / application overall. If you’ve taken other tests and feel confident in your scores, just apply now. Honestly, even if you don’t, I’d apply now and have a much stronger application (assuming you’re applying ED because NYU really is your #1 school, you just have to explain why 
).
But if you really think those subject tests will boost your app higher than everyone else in RD with those subject tests, then hold off on it. But really, it doesn’t make much of a difference imo.