Dave,

Usually you want to use a snap when the thing you are tying to is larger diameter. For example, the eye of an MS Slammer is very large, so you want to use a snap. If you tie directly to a big eyelet like that, it's hard to get the knot cinched up correctly.

I don't use a snap on the generic swimbait, or generic live trout softbait. The line ties are small, and for whatever reason it seems like I lose feel on the bait if I use a snap on those lures, even if it might make them swim more. I like to be able to feel the bait and feel what it's doing. Just a personal preference.

I've never compared the actions in the water with or without a snap on those baits to make a real conclusive comment about it.

I use a black snap, the ones that come stock on the Slammers. Something like this:

http://www.basspro.com/servlet/catalog.TextId?hvarTextId=30043&hvarDept=151&hvarEvent=&hvarClassCode=5&hvarSubCode=9&hvarTarget=browse