It's purely subjective. If you want the game a bit more floaty, 3.5 will do it. If you want it faster, then 5 will do that. All depends on how you want the game to play.
I recently jacked up the rear of my Sky Jump and it totally changes how the game plays. Much harder now to trap with the flippers. The game is way faster, which installing a new Wade Krause playfield also had something to do with it.