I would argue that for most shooters, it doesn't matter much unless it's way off. For the top end shooters, they may gain a couple X's at 600 with a well developed load. The x ring is 1 MOA. So is shooting a 3/8 MOA rilfe that much better than a 3/4 MOA rifle? Maybe, if you're good enough to be annoyed when you shoot a 10. If you're happy with 10s, and are dropping a fair number of points to wind and other mistakes, I don't think it's very important at all. Errors don't really add up proportionally - the largest ones dominate, and the smaller ones get lost. For most shooters, seating depth is a small error in XC shooting.
Standing and rapids? No. I don't think it matters at all. The target is huge, and either you're hitting or you're not. The load won't help you and it won't be what tanks you.
I'm admittedly a lousy XC shooter, but I've done enough of it to say with confidence that it's 95% shooter and 5% gear. You just can't buy very many points. A decent coat will get you more than a dialed in load. A good shooter can win with average gear without a doubt.
That's a big difference from F class or benchrest, which is at least 50% gear and setup. A lot of the shooter's skill is in preparation of the rifle and load development, and you simply cannot win with an average setup. A bad shooter with a good setup can do ok. A good shooter with a bad setup will do poorly. You need both to win.
I would also say that even for a bench gun, a tuner does not generally replace seating depth. Seating depth in my experience is much more impactful. A tuner helps to make subtle adjustments on a rifle that's already more or less under control.