We, sadly, were at the receiving end of a search by the TSA on our last trip, Oct 03 - and it wasn't very pleasant
To cut a very long story short, before we travelled I checked with both VA and our
travel insurance comapny (Nationwide).
Virgin Atlantic's view was that it would NOT be a problem to NOT lock the cases, but to put plastic ties on. Nationwide differed - as has been said before,
we would not be covered for any missing/damaged items if our cases were not locked. So when we got to Gatwick I again asked VA who said we should lock them. We did and had no problems.
Returning home was another matter. On checking in at Miami, with ties and padlocks in hand, we queried at VA's checkin what was the best thing to do. Blank looks all around, much conference in the background and we were finally told to lock our cases.
When our luggage came around the carousel at Gatwick, both DH's and my cases were unlocked and, worse, half-opened. The contents of the cases were in a terrible mess - lovingly bought presents smashed, two cans of Bud my DH couldn't bear to leave behind

opened and soaked into everything (they were so badly dented it looked as though they had been used as a football) A bottle of nailvarnish (which had been rolled in a flannel, then inside a sock inside a t-shirt for protection) had been smashed and blue (my summer toe colour

) was over new t-shirts and a christmas pressie blouse.
Gatwick VA didn't want to know, gave us a claim form and told us to look for the TSA letter inside. Yes, there was a small note from the TSA in each case saying, basically, tough luck but for the protection of US citizens . . . blah blah blah. Now, don't get me wrong, I'm all for increased security. But this was just blatant mishandling of people's property and I wouldn't have minded so much if I hadn't been so dilligent on checking with everyone what the heck we should do - lock or not

We were told by VA some time later, that even these 'approved' locks don't mean anything to the TSA and you are just as likely to get your cases ripped open with them.
So this year - and next - when we travel to the US, I will be locking my cases again because had we not, we would not have been able to claim for all the damage.
It's a tricky situation, one each individual has got to be happy doing the right thing, IMHO.
I tell you what, though, if they've screwed up my cases again I shall be back over there on the first plane, they will pay for the flights and all the damage caused

So there.
Good luck with whatever you choose.