I love tippet rings and use them on almost all of my leaders whether dry fly fishing, indicator nymphing, or tightline nymphing. In fact for nymphing, I typically use 2 tippet rings -- one just below the sighter and a second ring above the final tippet section. Tippet rings let you join light tippet to heavier tippet without a problem. They also eliminate many Surgeon's knots which are typically weaker. I have some 3 mm, 2.5 mm and 2 mm rings. I like the 2.5 mm rings for the typical stream trout size leaders and tippets. As others have noted, very small rings can be hard to handle and thread. Larger rings don't really have much negative impact, so go a little larger if in doubt.