I was always told to do double side the first 2 days to help your supply come in, then to stick with single sided at least for the first few months to be sure baby gets right to the hind milk.
That being said, we switched to double sided feeding once DS was on the 4 hour EASY (around 3 months). Because he was going longer between feeds he needed more milk, so I had to offer the other breast to be sure he was full. We still do double sided feeding.
I do agree with Martina though, I think it also depends on your supply. I had an over supply, especially when DS was first born, so one side was more than enough.