I initially wanted to go down the route of nothing at all - until I really sat and thought about it, and remembered my own childhood. I wasn't allowed ANYTHING - I can count the number of times I had sweets/chocolate on one hand for my whole childhood. Guess what? I now have the sweetest tooth you can imagine, and really struggle with self control. I think I was denied stuff for so long, I just went crazy when I finally realised I could make my own decisions.
So, with ds, I have decided to go down the route of allowing things, and teaching how to be responsible with it. I'm still very choosy what he can have (no artificial sweeteners, etc) but he is allowed a small amount of something, although I do not allow it every day. I am also careful not to use it as a reward for 'good' behaviour/eating his dinner, b/c I think that can set up unhealthy habits for later in life.
Here's a tip - if your lo hasn't had anything sweet yet, start with dark chocolate, rather than sweet milk chocolate. You know the dark 70% chocolate? I did that with ds, and he still likes it now - the benefits are that it's full of antioxidents, not packed with sugar, and rich so you don't need much. If you start with that, then the taste buds will get used to it as a 'treat'. I'm much happier giving him that than sweets.