I always use a secondary- but the answer is "it depends". On what kind of beer it is, how much sediment there is, how clear it is. If it's a stout or porter, clarity really isn't a concern. If you used a very flocculant yeast, it might be pretty clear after 10 days in the primary.
To me, a secondary is always a good thing- it allows the beer to get off the trub in the primary, ages it a bit and keeps me from drinking it, and a ton of sediment/yeast still falls out of suspension. But I always make amber ales, or pale ales. My beers come out pretty clear, too.
I've been following the 1-2-3 rule (more or less). One week (or more) primary, two weeks in secondary, three weeks in bottles. I always do at least one week or 10 days in primary, two weeks or so in secondary, but I do usually crack open a bottle after only a week or so. Just to check it