No sneak attacks shouldn't be a toggle. You are allowed only one sneak attack per turn. And when your rogue will get more than one attack a turn, you will be happy to choose on which of them you can apply your sneak attack. This is why it's a skill and not a default thing.
Let me give an example. You are level 6+ and behind an enemy who has 1 HP left. You won't waste your sneak attack on him if you can just kill him, move behind another adversary and use your sneak attack on a target where it will definitely be more useful. Add even more attacks per turn as you level up and you get the picture.
