When in the field for training, is putting up LSS practical?

You can look at it from a risk analysis perspective. 90% of your experienced personal are going to base the answer off of what you need to be concealed from. Most of the LSS is radar scattering and serves to prevent aerial observation. If the enemy has neither air assets nor battlefield radar capability, the advantage is negated and places you at a disadvantage. So, evaluate the METT-T to determine whether concealment or mobility is your greater defense against the notional enemy.
Just remember, the CSM is going to tell you to be practiced at it because it is a METL task, so you either put it up in the field or you're doing drills with it in the backyard. Pick your battles.

Now that many of us are training for operations other than Afghanistan I think it is important for units to at least have a basic knowledge on how to employ LSS.  Our primary mission is to defeat other Armies, whoever they are.

I can only echo what Chief Evans stated and from an old guy standpoint think it is needed in the field (garrison) for good training purposes in case you do need to utilize it, but it is a pain and always has been in both connotations.