Barrel Adjuster Strategy - What's the best way to use barrel adjusters?

I think you've described best practice for a couple of reasons. Firstly, cables will not tighten themselves, only slacken, so you provide the greatest adjustment to the bike owner; they have the full thread of the adjuster to use. Secondly, the only adjustment the owner can make in the first instance is to tighten the cable. The uninitiated rider can't slacken their own cable by mistake. Thirdly, if somebody else sees your work, if they notice this detail they will see it as best practice too and see that you took the care to get this right.

Reasons to do otherwise, that I can think of are if it's your own bike and you want to make adjustments in either direction because you know what you're doing, or if you are instructing someone remotely, such as online, you can tell them to set it halfway to begin with, which is a little more forgiving of their skills in setting the tension correctly.

That's it really, your options are to wind it all the way in and then set up the cables, or to set it not all the way in to make use of the adjustment.

Sometimes when setting up a particularly cheap component, there is so much flex in the brake for example that fine tuning without using the adjuster is a bit of a pain. But those should be rare examples.

It is fairly basic procedure to set up a v-brake or cantilever caliper such that it has good braking lever travel and you can also release the noodle or straddle wire to open the brake up and remove/insert an inflated tyre. However, if one finds this difficult to achieve, then setting the barrel adjuster halfway allows one to slacken the cable to remove the wheel. Sometimes noodles are difficult to un/hook if the cable is on the tight side.

Here is a minor strategy for rim brake barrel adjusters. My everyday wheels have rims with 24mm external width. My race wheels have 25mm external width.

When I got the latter set, I loosened the cable slightly using the anchor bolts. As things stand right now, when I fit my race wheels, I loosen the barrel adjuster by 4 quarter turns. Without any adjustment, the pads were too close to the rim.

This might not be a widely applicable strategy, but it does work. Disc brakes do obviate the need for this, although you might want to shim the rotors on one set such that they are in the same position as on the other set.

Otherwise, I would tend to agree that as a consumer, I might use the brakes' barrel adjusters as the pads wear, but I would otherwise prefer the shop to use the cable anchor and tension. I may have used the downtube cable adjusters on my bikes with mechanical shifting once or twice while in motion, but it is sometimes hard to get a good grip on the adjusters, and I would not have needed to use them if I had adjusted the derailleur properly to begin with. (And yes, the one or two times I used them, it was my work and thus my fault.) I'm not sure there's any real advantage to the downtube adjusters aside from the fact that you can access them while still moving.

My personal strategy is to leave some room to loosen the barrel adjusters and then make the cable as tight as possible so that the brake pads are against the rim. Then use the barrel adjuster to loosen up the cable and provide just as small a gap as possible between the brake pads and the rim.

This has a problem that you don't have as much room to adjust with the wear of the pads, but I find it to be a good trade off for dialing in the correct cable length on new pads. All my bikes have v-brakes or cantilever brakes, so this may not work well with other braking systems such as disk brakes or caliper brakes.

For shifting I try to get the derailleur positions as close as possible with the barrel adjuster in the middle, and then use the barrel adjuster to trim as desired until shifting is dialed in.