After my experience with the post-blocking growth of my cardigan, I see the virtues of knitting a swatch and recording the gauge before and after washing.
My Tahki New Tweed is the first of my yarns to undergo this treatment. Gauge before washing: 19stitches and 25 rows per four inches. Gauge after washing: 18 stitches and 27 rows per four inches. Wow! Is it any wonder why I always struggle with sizing and fit? If the knitted fabric changes that dramatically once it's washed, imagine how that translates into a knitted garment!
So, wash your swatches.