I think it comes down to whether or not you can open hirelings mail on another toon and have those mats scale with person opening. I did a few checks. You can ignore your mail when you log onto an alt, and you can open that up on another. I did not check to see if the mats came out higher by doing this.
For instance my necromancer is new and has less than 50 in trade skills. I have some tier of hirelings on him. If say I grind up blacksmithing to 50 by mass deconstruction, that unlocks all tiers of hirelings and the gathering detection range. For now most of these alts just sit there with only craft skills specified in addition to a few 1-point allocations in the categories of skills I wish to slowly grind to 50. Each writ turn in generates experience too, so I have been able to passively up skill lines by doing writs on inactive characters over time.
Where it stands now though is if I have an alt with say 50 blacksmithing, I will have 10/10 in blacksmithing with 3/3 in hireling. If I go to 1/10 in blacksmithing with 3/3 in hireling, and open his hireling mail on a 50 blacksmith, will I get blacksmith mats scaled to 50, or blacksmith materials scaled to 1/50? If it is the former, that is 9 points I get back, which would be very helpful when I dust it off and resume levelling.
From a writ standpoint, the writs seem to generate gold and exp based upon your character level. But I will only see master writs on the 50 skill toons, which is argument enough to hold 10/10 in something like blacksmithing as long as the toon is inactive.
As I grind to 50 though on these alts, I do unlock passives. For instance I can go 3/3 on potion duration from Alchemy because I ran it up, and get a nice boost.
27Jun2019 Update- if you open the mail, it will open at the tier you have. You can leave it unopened on all low level toons, and just open it on the high level toon. Gathering associated with that profession will track the tier level of the skill. This was driven home to me when my crafter in JC needed silver and my main had a lower JC skill and was not gathering that type. Writs seem to reward better if you have a higher tier skill. So the guys at 50 seem to have a better chance at goodies. Also you will tend to at some point glut in end game materials, and so it is better to have everyone at the higher tiers to exploit that big supply when they do writs, rather than run down say your 30-something mats.
If you are passively crafting, you can just dispense with Keen Eye. You are not gathering. 3/3 hireling is nice, but what I am seeing is the lion’s share of my mats come from surveys now. If I open all mail on all toons I maybe have to refine 2 stacks (20 resources) per type. But after I run surveys, I come back with 10+ stacks to refine. So if you are point squeezed, maybe dump the hireling.
Crafting extra of the type is great. You will get a daily for it, and over-produce. Just keep the extras in inventory, and when that one comes up again, you just turn that in. Saves you resources.
If you are leveling up a skill, then it helps having points in deconstruction yield. I think you can leave the crafting tier be low if you like, but getting more from the mountains of stuff you deconstruct is nice.