Keep it just under 33% (e.g. 32.81%). There's a threshold (plus more at higher levels) where POPs start to get a lot more ticked off if you cross that line.
I don't believe in bleeding my POPs dry. But, I do believe in relieving their money from them, all the same. Instead of taxing them directly (at which time they get mad at 'the man' and get militant), I'd much rather they invest, and buy stuff - which I make money on (bonds, tariffs, etc). That way everybody's happy... I get lots of money, low militancy, and high productivity... and they get all their little telephones, planes, automobiles and what not.
Bleeding them dry is just asking for a Bolshevik revolution. Not my style. I like the capitalism technique. And I tend to swim in a lot of cash, with advanced industry, tech and infrastructure.
Work WITH the people, not against them.
Btw, when ABSOLUTELY necessary, I think the next 'threshold' is 50%, so tax at 49.xx% if required. Sometimes you need it, before your economy gets up to speed, and usually the rich can afford it.