This is such a commonly debated topic, it's nearing "troll status" applied to anyone who might bring it up. You'll likely find a lot of folks who will tell you it doesn't matter much, or you'll find others who'll say it's absolutely critical. Personally, I weight my reloading for precision like this: bullet weight & length fitting my twist rate and velocity = 50%, finding a well suited powder which offers a forgiving powder window at my desired velocity (near max) = 30%, Seating depth = 12%, and everything else you could possibly imagine = 8%. When I develop a load, I start based on the rules of thumb you'll see below and pick a "standard" base-to-ogive length, work with a few known good powders, then when I find a happy charge weight, I manipulate my seating depth to try and close down horizontal.
For handgun, seating depth isn't irrelevant, but not many folks shoot a handgun well enough to notice the difference between seating depths on the target. For revolvers, I like to seat to cylinder length whenever appropriate, else I'll focus on crimping in the cannelure or crimp groove, OR enough seating depth to ensure I don't jump crimp. For pistols, I find a good powder charge and work in and out a bit to see if it moves on target, but I'll typically load pretty close out to the lands.
For rifles:
Seating depth, or rather the desired bullet jump, depends upon a lot upon the type of bullet chosen. Monometal bullets like the Barnes TTSX or Hornady E-Tip like a long run, so these traditionally shoot their best when seated to 50-70thou off of the lands. Secant profile bullets like VLD's tend to need to be rather close to the lands, or preferably kissing or even jammed, to help ensure they enter the rifling true, since the ogive doesn't flow smoothly into the bearing surface as it does in a Tangent ogive bullet. Dual Cavity or segmented core bullets like the A-Frame or Partition tend to act like the monometals, and prefer a lot of run up into the rifling, so 50-70thou jump. Tangent "match" bullets are incredibly forgiving, and might hit their sweet spot at 5 thou clear out to 40thou. Hybrid bullets blend the secant long ogive with a tangent curve at the bearing surface junction, so these tend to be more forgiving, although not as much as a traditional tangent ogive match bullet, so I generally find my sweet spot between 5 to 25thou for these.
For most of my loads with non-mono or non segmented core bullets, I'm between 5-15thou off of the lands - starting whenever I first start loading for that bullet, and rarely if ever actively chasing the lands as the throat and leade erodes. For VLD's, I'll be 5 thou out to 5thou jammed - jammed in bench rifles, out for hunting/field rifles.
