How were soldiers assigned?

[Phaichtos]

You also need to remember the concept of patronage in Roman society, which also could have big part to where you might be assigned to.

There were several letters from persons to centurions, commanders and governors trying to secure positions for their clients, Pliny Younger letter comes to mind first. So, I bet that assignment depended a lot who was your patron and who he knew. So, instead of being sent to some dry outpost near Parthia, one might well be able to secure comfortable position in commander staff etc. if his patron was influential.

True though I would imagine this could be quite rare for the rank-in-file, especially after the army began to be dominated by non-Italians...