For parents sending high-school seniors to college in the fall, here's a surprising financial tip: Contributing to a 529 plan even just months before the first tuition payment is due will qualify the account owner for a tax benefit in many states.

Adding to a 529 can lower the state taxes you owe—under certain conditions—in 34 states and the District of Columbia, according to a tally by college-planning website FinAid.org.

Make...