If I absolutely had to be in the Imperial Guard (and chances are, I would if it was the real 40K universe...): I'd hope to get something with at least a little survivability...
But, who are we kidding?
We'd all most likely end up as ground-pounding grunts.
Oh, and if I recall correctly, Commisars are technically Departmento Munitorum, which would be like the U.S. Department of Defense. So, attached to the Guard, but not actually part of the Guard.
And titans are AdMech...
Okay, if I had to choose a job in the Guard, I'd like to be a Comms guy, I guess.
'At two minutes to eleven, opposite the South African Brigade, at the eastern-most point reached by the British armies, a German machine-gunner, after firing off a belt without pause, was seen to stand up beside his weapon, take off his helmet, bow, and then slowly walk to the rear.' -John Buchan
I can't help but think that every time something is proven, a little bit of the magic of the world dies.