Keep 1 wound ringwraiths alive. Wraiths like Kamul seem good but without the ability to use their will as fate they seem kind of easy to kill with only 1 or 2 fate for 120pts investment.

What are they dying to? Their main worry tends to be against Legolas who can just snipe away, hoping to get lucky with a wound roll or simply invest some might to achieve a dead wraith. Other than that, D8 and a huge number of in-the-way rolls from models and/or scenery tends to keep them safe from shooting. In combat (on fellbeast or possibly horseback) they should ideally just fight transfixed heroes or small numbers of basic troops, some of which will be lying prone, so not much of a risk either way. Although I guess the amount of axes some troops have could make the D8 a lot easier to crack.

well, most Nazgul don't take risks. Their magic has a 12" range, so they usually hide behind two or three ranks. Even Legolas would need 6/4 to wound them. Regarding Fighting Nazgul like Khamul, you just need to pick the right fight and avoid unnecessary risks.
But really, you have to see what your opponent has and decide what's best for your nazgul, considering what is his role in your army in the first place.

Keep them out of combat. With some exceptions (Wiki, KoU, and Khamul), your Nazgul should not be fighting on the front line.