There are still lots of unanswered questions in the theory though..
First off, as Simon was hinting at - how much of a difference in spread would there need to be before it becomes noticeable in your groupings?
If you have a pellet that travels faster downrange and has a tight spread, that's a good indicator for a good choice in pellet, but what if you have a pellet that's fast downrange but doesn't have a tight spread versus a slower pellet downrange with a tighter spread - which one is theoretically better?
I have no idea what those answers might be
There are a couple of things that are pretty certain though (unless the last few hundred years of ballistic theory is wrong)... if you find a pellet that groups well, that's an obvious candidate for the batch you should buy. But if you find two or more batches of pellets that group well, pick the one that travels the fastest downrange. (or more accurately, the one that loses the least velocity downrange i.e. the one with the highest BC).
If you're crap at shooting groups and can't reliably tell whether one batch of pellets is better than another - if you pick the one that has the highest downrange velocity and the tightest spread, you're probably not far off finding the best batch and it's a lot better approach than just flipping a coin to choose.