If you are planning on having SRS within several months to a year, it doesn't seem worth it because of the cost. Most surgeons don't give a discount, and some won't perform SRS if you've had an Orchiectomy. To compliment Nero's comment, if you have an Orchi the tissue may shrink, which means the SRS surgeon has less material and thus your results (vaginal depth) may not be as good.
However, if you're not planning on having SRS for several years, or at all (as in my case), then an Orchi might be a good intermediate (or final) step which can eliminate testosterone production and prevent some of the risks of long-term HRT.
Jenn