Sometimes leaves can pack down into an air tight layer when they are wet and half rotted. That's not good for the grass, obviously. I would either remove them and compost them, or run a mulching mower over them a few times.

In the UK, they need to be raked off the lawn - winters tend to be chilly and very damp, and if you leave them, you'll have lots of bare patches in the lawn next year.

I agree with Bamboo but it does depend on how many leaves. if it is only a few and I mean a few then probably not but if most of the grass is covered then yes it is

Depends what you mean by a few though Seaburngirl - there's a bald patch in front of the apple tree at the end of most winters in one of my client's gardens from the 'few' leaves that sit there all winter, drives me mad they don't pick 'em up, I'll have done the bulk of them myself, earlier, then they leave those, then I have to do lawn repair...