I have always tied my line direct to the lure, but recently the fish have been wanting different things every day so I have been switching lures allot. Could I use a snap hook for crank baits and swim baits or will it negatively effect the lure's action

The snaps always seem to make the bait run differently. I think you are just gonna have to keep tying...

It depends on what you are throwing. A popper? Not really too much. A Texas rig? It depends on if it is rigged correctly. Inline spinners and spoons? I have been going with snap swivels. The only problem with that is is if you are going with small and light Mepps spinners the snap may make it too heavy. Phoebe tins tend to twist up the line. It sometimes depends on if the lure has a split ring or is just an eye on the nose.
I just recently got back into the sweetwater game after many years. I am learning all over again. If I am wrong on any of this don't bash me, it is just what I am trying to figure out again after alot of time off.
You are better off testing it all yourself. No body has the same setup here so you may have to figure out what works for what you have. That is half the fun anyway. Everyday I learn something new whether it is fresh or salt water.