I would consider two criteria when deciding whether to use a snap or not.

The first is knot strength. A snap can improve knot strength when the line tie is very thick (like on an MS Slammer). A snap can also improve knot stregth for baits with strong head movement where the knot would chafe against the line tie during the retrieve (like Triple Trout).

The second thing to consider is bait action. A snap ensures that the bait pivots on the line tie consistently. To understand this, tie on a zara spook some time and cinch the knot down tight. Try walking the dog with the knot pushed up toward the top of the eye. Then push it down below the eye and try. You'll see the difference in the action. This is why they always suggested a rapala knot on the back of the Rapala package :) Big baits, especially wood baits are the same way so you can use a snap to eliminate the inconsistency.

Here's a list of baits where I use a snap...

MS Slammer
Monster Jack
Triple Trout
Live Trout Hardbait
Castaic Trout Softbait or Hardbait
Any of the AC Plug type baits
Castaic Platinum

I'll use a #6 owner hyperwire split ring on the nose of the Live Trout Sofbait sometimes also for subtlety but that's the only time I tie to a split ring on a big bait.

Here's a list of baits where I don't use a snap

Hudd
Stocker Trout
Optimum
Any of the Mattlures baits
any boot tail swimbait

Like anything, you shouldn't get stuck in some mentality like 'I am always going to use a snap' or 'I am never going to use a snap'. It's always best to try with and without and just see what works better. A recent example of this for me was the Platinum. I thought that the bait would not need a snap at first but after using it I realized that the action was easily affected if the knot got crooked on the eye of the bait so I started using a snap on that bait to ensure that it swam as straight as possible.

-Rob