on our clear streams in the Cape, if the tippet is lying on the water, the fish will mostly refuse the fly.
Degrease and get the tippet to sink, and the fish takes the fly.
We have had refusals, with a floating line, gone a X down, and the thinner tippet also floats the fish still refuses it, but degrease the tippet and get it to sink, the fish takes it.
Some of us believe that the new tippet, even if it sinks, has a "glossy" finish, which deters trout.
So all tippet, even flouro carbon gets the treatment, to reduce the "flash" component.
If you can't see it, the fish should also have a smaller chance to see it as well.