Sure, but if someone cuts your suitcase open with a knife it'll be much more obvious than if they used the master key to open it and then zipped it shut again.
I've been through the training for airport security officers (not in the US, though) and that's one of the techniques they taught us. Also most baggage locks are ridiculously easy to pick with basic tools (literally a 15-30 minute lesson) even if you don't have the master keys available.
No, for instance anything with a zipper can pretty much be opened with a pen instead of a knife. Just poke through and it will unzip. Depending on the zipper quality you should be able to just rezip it.
