Let's talk about the wonders of superwashed fibre. It's extra smooth and soft, and, best of all, it won't shrink or felt if you accidentally throw it into a hot washer or dryer (not that you'd every really do that with the beautiful garments you knit with such love and devotion). While these characteristics are fantastic, there are other outcomes of the superwashing process that you should keep in mind when using a superwashed yarn. First, dyes will be a little less stable on superwashed fibers, so just keep in mind that you should expect to see more dye released into the water used for your first wet blocking. This will be especially noticeable when using a yarn with a very brilliant or intense color. Just remember to add a touch of Eucalan to the warm soaking water, let the object soak for at least 30 minutes (no aggitation, just some gentle poking during the soak, please), and then keep rinsing with tepid or cool water until the water runs clear. If you're not sure if the water is running clear, roll the piece in a white towel and press firmly. If the towel shows color, put the piece back into the water to continue rinsing. Even though you may see colour coming out of the yarn, you shouldn't notice a change in the yarn color after this over-dye rinses away. You, also, should not see significant amounts of colour rinsing out of the yarn in subsequent washings.
Another characteristic of superwashed wool can be a tendency to grow a bit during wet blocking. My best advice is to take this into consideration when choosing a desirable gauge for the fabric of your project. Plan to knit to a slightly tighter gauge than you want in the finished fabric, because you can expect the yarn to relax and open up a bit during blocking. It's hard to say how much, but you might see 22 st/4 inches grow to 20-21 st/4 inches. Of course, the best course of action is to do a gauge swatch, block with the method you plan to use (and I do recommend wet blocking to wash away any over-dye, even up stitches, and enable proper setting of shape and dimensions), dry the swatch, and then consider the look of the fabric and the gauge measurements.
Superwash wool is a great fibre for items that are going to need frequent washing or that run the risk of being accidentally tossed into the machine with the rest of the laundry. Knowing what to expect from your beautiful superwashed yarn and how to deal with its distinctive "personality" traits will help you avoid surprises and keep knitting happy.