I like to leave a space because I consider the worktop as a potential movement .This way I can get a good joint for silicon. I know some will say that no way the worktop should move so this is only my preference.
I tend to make sure that the worktop is completely level and tile straight off of it. I like to leave the thinest line of silicon that I can so keep gaps to an absolute minimum.
As previously stated, "there is no reason why the worktop should move."
I always leave a space, just one or two mm, because even though it shouldn't, the worktop could potentially move. Besides, it's in the regulations to leave a space over here, so I don't really have a choice.