I usually use the finer tippets to get the smaller flies tied on and layed out on a cast. I doubt a 2x tippet will go thru the eyes of a size 18 dry hook. I don't switch to the lighter tippets just because the fish are spooky.
Here is a chart I picked up a few years ago. Of course, the diameters are probably regular mono line.
X Rating Diameter Fly Size
0X .011 2, 1/0
1X .010 4,6,8
2X .009 6, 8, 10
3X .008 10, 12, 14
4X .007 12, 14, 16
5X .006 14, 16, 18
6X .005 16, 18, 20, 22
7X .004 18, 20, 22, 24
8X .003 22, 24, 26, 28
For example take a size 6 hook, divide 6 by 4 and you get one. Add one to that and you get 2. So for a size 6 hook use a 2X leader. Of course the chart says 1X or 2X will work - which is right? Well both are. The formula is a guide to get you close. A lot of people make do with a spool of 2X, 4X and 6X tippet. Check it out, 2X will do hook sizes 6, 8 and 10; 4X will do hook sizes 12, 14 and 16; and 6X does most everything else. Does it matter which tippet you use? Really? Yes it does. You might get a 2X leader through a size 16 fly eye, but you won't get much of a dead drift. The tippet will drag. Adjust your tippet to your fly and catch more fish.