AT NW LAX check in, do the TSA check the bag at check in, or after, so I can lock my bags after they check??

At any airport, once the TSA completes screening on a piece of luggage, the owner can not handle it...this would defeat the purpose. Normally, you can ask them to lock a bag after they complete their screening.

At NW-LAX, the TSA screens your baggage after you have completed check in. In my experience, the TSA crew is s-l-o-w, and given to small talk with one another, when they should be occupied with processing the luggage more quickly.

Contrast this with UA at LAX, where (often) a TSA agent will take your bags while you are waiting to check in, and will have them completed before you finish checking in.

At UA, the TSA baggage process often adds zero time to your check-in, whereas at NW, it adds 10-20 minutes.

This makes me wonder how much say the airlines have over the TSA working alongside them. In my experience, UA's customer service far exceeds NW's, and that seems to be paralleled in the TSA's efficiency and attitude at those airlines' outposts at LAX.