So now the answer is: "It depends on what brand you're using, whether it's the top-quality stuff of the cheap." If you remain unconvinced that spending the money on artist's quality paint is worthwhile, yet get annoyed by a color shift, buy a small tube or three in colors you often use and do a comparison.
Does a color shift matter? I've never been bothered, but I know some people find it exasperating. But perhaps its because I'm happy painting on the dark side...
Artist's Quality vs Student Paints
Art Supplies List for Acrylic Painting