i do three things when strip-piecing for tube-quilting and similar methods.
1. i prewash the fabs (just a personal preference.)
2. i starch the heck out of the fabs.
3. i do not cut my strips across the width of fabric. i cut them "up", starting half an inch in from the selvadges.
by changing to the non-traditional direction when i cut the strips, i'm left with strips that don't stretch along the seam line. no fabric rainbows ... even if i don't remember to alternate the beginning between tops and bottoms.
the bonus: when i cut up the strips and start using them together, i do have that little bit of stretch between seams. it comes in very handy when patches are slightly off-size and i need to ease them to fit. :-)