Presentation isn't the problem with nymphs that it is with dries, so taper makes little difference. The main thing is to have the nymph in the bottom layer of water. In shallow streams a weighted leader or nymph can accomplish that. In deeper water, a sink tip or sinking line is better. No matter what, I see no reason to get a line tapered for nymphs. Shoot, a level line would be fine with them if you could stand to cast it.