One help for underspun singles ... before you let it wind onto the bobbin (or wind it onto the spindle), hold the length of new yarn loosely so it can ply back on itself. This shows you what the balanced 2-ply would look like. If you are shooting for a 2-ply, then use the "ply back" test to ensure your singles have the twist you want for a nice 2-ply in the "ply back" test.
If you are shooting for a 3-ply, the singles need a little less twist -- a 3-ply at the same twists-per-inch as a 2-ply takes less twist out of each single (since it's taking it out of 3 singles, and the 2-ply's taking it out of 2 singles). So you can go for a "looser" looking result in the "ply back" test and still get a nicely spun/plied 3-ply.
Now me, I always forget that the 3-ply needs less twist in the singles, so my 3-plies usually have a fairly high twist (since the singles do, too). Someday I'll remember to put less twist in my singles-for-3-ply.
But you may find they all balance out in the wash. The water bath lets the twist in the singles and in the ply revert to stable (or try to), which often makes what appears to be an over-plied skein be a balanced skein -- and what appears to be a balanced skein to be underplied.
Luckily both over and underplying are relatively quick to fix -- just run it through the wheel again, either taking out plying twist or adding more, as needed.
(posted by me this day on livejournal\spinningfiber)