As cabincrew, we are told to ALWAYS lock our suitcase. The TSA are aware of this proceedure of ours and in some airports our suitcases are either scanned in front of us by the checkin area so that we can open it for them if they want us to or they will come to the aircraft and ask us to open it there for them.
However, I have your typical Delsey suitcase with the latch on the front with a combination. These (and the likes of the samsonite ones) are really handy as the TSA know how to open these. I have regularly found a little note in my suitcase saying that it has been searched by the TSA. However recently I found a great big mark by the latch where it had obviously been forced open...again a note left by the TSA!!! I think some airports are just more careless than others!!!