No, I think you asked this on another site as well, whether you did or didn't makes mo difference: the union discount is available but like all discounts, it isn't available on all sailings. If it is high season or the ship is almost sold out there are few discounts.
Back in 2008-2009 the union discounts were substantial between 10 and 30% on several sailings. Of course you had to be able to book well in advance, but it really was a great deal. So much so that I rejoined the Musician's Union just to take advantage of these once great discounts.