I agree with Fred on this one. The sink tip should pull your tippet down to the water column your looking for, then the current will do the rest when it comes to leveling out the fly (assuming your fly isn't some type of floating foam beast). If your fly is floating up to the surface with a sink tip then you have some real buoyancy issues.
Sometimes I like to put a longer tippet (6' or so) on my sink tips in really clear water just to make sure that spooking isn't an option. It probably doesn't do anything but it makes me feel better inside.