I think I shift 'philosophies' based on the scenario -- and I'm not familiar with any of the Bulge scenarios, so I can't get specific on yours.
If the scale allows for a lot of separate artillery units (Anzio 1KM or 2KM, TWIN,...) I play differently than, say, Road to Moscow, where there are only a few artillery units, comparatively. And it depends on how the scenario is designed - in some, micro-ing the artillery does not matter, even if you have the units; and who is my opponent, some just wheel 'em up, point, dig and that's it; others who are more detailed, and I think, difficult; and then there's Telumar....
Short version - most of the time I play with artillery units set to overlap the infantry/armor across the line, and deep (where do I think the front will be in 2 or 3 turns). I set most of the artillery in D, E, F is support; I may reserve heavy artillery for direct fires (see below); I use TR when I want flexibility in moving artillery during the time and/or support fire at reduced supply cost.
Longer version -
I do check my units - Man,I hate those mixes of units where 105s are jumbled with 155s and/or 120mm mortars. Range matters, so I try to locate supporting artillery so the maximum amount of tubes bear on the likely zone.
On defense, I mostly go for digging in artillery support to get maximum supporting effect. Exceptions are the heavier guns -- 150/6in guns and larger. Their shell weights make these very effective on breaking down entrenched levels, knocking opponent artillery out of TR,D,E,F - and disrupting likely attackers.
With the new supply rules, I found it important to limit support so that units were not always deep cherry red; sometimes, ya gotta just blast away, but you do get more effect from a unit with higher supply and more readiness. Again, this depends on scenarios - in Anzio 1KM, playing as Allies, I found myself resting 1 out 3 of the 105 batteries almost every turn.
And here's where support comes in, if support levels are Army,Force,Free and the designer has not gone beserk with color-coding units, the artillery units will support everything it can within range, and you wind up burning a lot of supply in disengagement fires, counter-battery, etc for actions or units that I would consider marginal. You're just not shelling against the those infantry regiments hitting your front lines, you shelling everthing that triggers recon and that's a lot of triggering.
So rest helps focus, but resting means that the artillery is more vulnerable to air and/or counter-battery and/or a break-through assault, even when in a hex at 70% or higher entrenchment, the losses are higher than when that unit is in D, E, F.
As Curtis points out, setting a unit at TR allows freedom during the turn to later dig in, or move, or directly bombard some unit. And if the turn ends early, you are in support.
As I understand it, TR does not deliver the same support as D or E or F -- TR delivers one-half the amount. So a TR set at LL will be as effective as D set at ML. Still, that does add up.
AND it only burns half the supply -- this really matters when supply is constrained.
This is for artillery -- air and naval are different cans, and different worms. Oh, and I do not always do as I say - again, scenario and opponent and game-turn-situation matter.
As for those long-range K18s of Telumar's, well, that's why Wallis invented the Wellington - damn fine tactical bomber.