A small amount of vertical play in a lockback is perfectly acceptable, and even preferable in some cases to certain people. It can actually make the lock a bit more secure, as it gives the lock a little bit of give before failure.
Say you have a little bit of lint in the lock well, a knife with play is likely going to still lock up, while one with no play at all may very well fail to lock up, or worse, close on your finger.
Riveted FRN knives can be tightened. It's been explained a few times around here. The general consensus is to insert razor blades on each side of the pivot area, hammer the pivot pin LIGHTLY until the play is gone, and then remove the razor blades. This usually fixes any play in the knife.
(Disclaimer, I'm not an expert, and I personally have not done this as I haven't needed to. This might not be the correct procedure either, I'm going from memory)
In the end, it all is a matter of preference. If you don't like the play, send it in for repair. But if it's like you describe, it's not something I'd worry about too much.
