I think it should be mandatory.
Anyone who wields that much power needs to be thoroughly vetted.
I mean, fer Christ's sake, even a Uber driver gets a background and credit history check.
But we can't demand that a POTUS turn over his financials?????
Actually, it should be a choice. Either release your tax returns, or if elected president, you put everything into "blind trust"
And when I say "blind trust" I mean the very definition. Where your assets are put under the exclusive control of an independent body, whose duty is to liquidate the existing assets (without losing money) and acquire assets the president is unaware of.
This way any presidential decision may or may not help him financially, since he has no clue where his money is.