I just found the WJ III Assessment Service bulletin #11, which states "The W difference is the value from which standard scores, percentile ranks, and relative proficiency indexes are derived."
You can read it here:
www.riverpub.com/products/wjIIIComplete/pdf/WJ3_ASB_11.pdf

Since it's all done by computer, it shouldn't be a big deal to ask for W scores from my tester, right?

Also, from what I've read it sounds like the tester can choose between age or grade norms.