Though I still mix the paint (gal.s into 5s, 5s into each other), color is not so major a reason as it used to be. Yeah, a couple degrees off-tint and it can drive you nuts, but my D-E rep took me to lunch a number of years ago when I still flirted with production, and he gave me the ultimate reason to box paint:
Though the technology has advanced in leaps and bounds to match paint tint virtually molecule for molecule, you can't keep a good fear down. Yes, there are lousy dealers and some lousy tint machines out there continuing to make a bad name for the trade, and the labels themselves often tell you that "only God is perfect", perpetuating the idea that color matching ain't all that close of a science, which in isolated incidences may be true...But the reason manufacturers jump up and down insisting that you box your paint is for
consistant sheen...
It seems that, from batch to batch, there is not a coatings manufacturer on the planet who can match enamel content and the amount of sheen between any two of those batches.
This includes stock colors...
Folks will not notice a slight difference in color, if it is actually slight. They may regard it as a shadow or the light playing with their own eyes. But there is no lower limit on sheen; If there is a difference and a light source, it WILL be noticed.
So, if you
don't box your paint, you might want to make sure the batch numbers match...