1858,
At the Steel Safari the shooters have to find, range, and then engage the targets within a set time period. It is not uncommon for a competitor to be unable to find one or more targets. In that case, they couldn't necessarily shoot them left to right or close to far or whatever. It is the shooter's responsibility to communicate effectively to the spotter what target he is shooting at, or his shooting order. Since he can't look for target until he is on the clock, that communication must be on the clock too.
Realistically, an experienced spotter does not need to be told what target the shooter is engaging: he can visually line up with the shooter's rifle and spot the target. Although in a match environment, the spotter should use magnified optics and electronic earpro so the shooter gets full benefit of any doubt, when spotting in local matches I just use my MkI eyeballs and ears (provided the shooter has a suppressor) and can spot just as well as with binoculars.
From the shooter's perspective, it is in his best interest to use a target order that gives him the most wind feedback with the least chance of missing. For targets of the same size, this usually means close to far. So if you have a line of 10" targets, he can shoot the first one and a full wind value may not push him off the plate. Then he can use the information from the actual impact point on the plate (hits/misses spotted through his scope as he shoots) vs. his point of aim to get wind information for the further targets.
If the targets are different sizes and there is a small target closer than a larger target further (IE a 1 MOA at 300 and a 2.5 MOA at 400), it may be in his best interest to shoot the 2.5 MOA target @ 400 because he can have more wind error and still hit the target, and use the impact location on the larger target to figure out his wind call for the close target.
One exception to this kind of ordering is for convenience and overall efficiency of motion. Shooting from the field, it is not uncommon for only a subset of the targets for a position/stage to be visible from the most stable shooting position. For example, you might only see 4 of 6 from one prone position, and then the remaining two are visible if you move over 6 inches (due to intervening terrain or brush, etc). Or maybe three targets must be shot from sticks (or sitting or standing) due to terrain and target placement while the remaining ones can be shot from prone. In these cases, I will shoot in an order to minimize movement and wasted time, which means shooting all that are visible from the first position, then moving and shooting the rest.
One other related exceptional case is where there are targets of substantially the same difficulty (say 6 targets spread across but all in the 380-425 range regime). In that case, I will shoot an order that minimizes sight picture movement shot to shot or is easier to remember, so that might be left to right instead of easiest to hardest (or close to far).
With regard to how to actually hold for wind, you have roughly three methods: reticle, knobs, or "Kentucky" (ie holding off 6", 9", whatever). When engaging multiple targets with only one shot apiece and the wind hold is unlikely to be the same amount, using the knobs is too slow and error prone. Using the reticle marks to hold the angular correction from your dope card is the best way to do it for field matches because reticle marks are objective and hard to get wrong. Even with known target sizes, if the orientation of the target is unusual (like a 10" square hung as a diamond) it can be difficult to figure out on the fly. Once you get to where you are holding off the target more than a few inches, using "linear distance" holdoff is problematic because holding say 10" off the side of a plate is much more difficult to do visually with no reference markers in space than holding 0.75 mils right of the center of the plate. The only exception I make to this (using reticle holdoff in angular units) is that for wind calls that would still be "on the plate", I use the linear distance as a sanity check. For example, if I know the target is about 10" and a full wind value is 6" (and I judge less than about a full value), I know I can hold just inside the edge of the plate (maybe 2" from edge) and even a full wind value would not push me off the other side. Anyway, this tradeoff is reflected in my range cards by listing a full value of wind in inches only when it's less than 12".
With regard to target sizes- at our matches we have a variety of target sizes and formats. We might have squares/rectangles, diamonds, and circles that all have the same surface area. Or we might have 6x12 or 10x10 or 8x12 targets. With no guarantees about target size and configuration, it's best not to rely on it.