Mostly, the out-of-true up/down wheel simply makes your ride less pleasant. Out of true side-to-side will cause the brakes to drag, if severe enough, and make them grab a bit on light/moderate braking, even if they aren't dragging normally. And either will make the bike harder to handle at high speeds (especially those 50mph downhills).
In theory, an out-of-true wheel probably affects spoke life, and a wheel, once out-of-true, will tend to get worse over time. And, of course, if a wheel goes suddenly out of true it may indicate that a spoke is near failure.
But none of this (other than the handling problems, which you can evaluate yourself) represents a real hazard, and there is no great harm in continuing to ride on the wheel if it's not too unpleasant to do so. It's not like a loose crank arm or some such where failure to fix it immediately can cause serious additional damage.
All that said, a decent quality wheel in good condition, once trued for the 2nd or 3rd time (ie, fully "broken in") should stay true for years, if you don't subject it to any severe shocks. So it's worth paying to have it done (or learning how to do it yourself) to help assure years of pleasant riding.