I'd say whenever you feel comfortable doing it ?
Defense spending : There's no point in cranking it up to 100%, once you have enough soldiers it can stay around 60%, 70% if you're sure you don't need the pops elsewhere.
Same for admin spending, once you reach 80-90% admin efficiency there's no need to keep it at 100%
etc. for education and other sliders.
Yes I realize I answered the question backwards in a way, the idea is that at some point your income should rise because of techs and conquests and your spending does not have to keep rising, which means you'll have a lot of surplus, and then you can lower taxes.
Tariffs are another beast, they can be very useful early game to balance the budget, but they'll prevent your pops and factories from getting the goods they need which may slow your industrialization and raise militancy. But then sometimes it's what you want...