Byron,

usually when I tie a pattern with an egg sack, I will use a small ball of dubbing for dry patterns, or one wrap of micro chenille for wet patterns. most of the bugs that I have observed on the river have a small ball shape of eggs attached to the rear of the abdomen. im not sure that the actual shape makes a big difference especially when you look at a lot of the classic patterns that used a wrap or two of floss or tying thread to represent the egg sac.

my thoughts are that whether the bug your trying to imitate is a dipper or a diver is much more important. if the bug is a diver, I don't worry to much about an egg sac on my dry pattern, usually I will switch to a soft hackle with an egg sac on it. if its a dipper, I will fish a dry with an egg sac. im not sure that this works for everybody, or that if its even correct, but it works for me.