I've switched machines in the middle of a quilting project, and I can tell where I switched (subtle difference in the stitch formation). But this was for quilting- for piecing, it shouldn't make any difference at all, as long as a quarter-inch is a quarter-inch!
I've been known to switch thread too when FMQing (I do it all the time when piecing...as long as the weight is close to the same it is fine) When I've done that, even "lay eyes" can tell the difference, as a thicker thread stands out more, but I don't really care.