The heuristic I generally use is "use parentheses as needed, but rewrite if you find that you're needing to use square brackets." Why? Thinking about it, I believe this is because I see parentheses all the time in professional texts, but almost never parentheticals inside parentheticals.

But as I verbalize this heuristic, I suddenly feel like it might lend the writing a certain charm or desirable style to defy convention and double-bag some asides. Hmm.