Seriously though, is the difference that big of a deal? Most people just want the stuff to work and not make a mess. Sure, you might get a slightly better mix doing it yourself, but we're talking about filler here, not a science project. Sometimes good enough is actually fine. How much time are we really saving versus the result we get?
You mentioned it's not a science project, but the mix really does matter. If the filler separates, it won't work right and can make a huge mess. That means you waste money and time cleaning up. Doing it yourself ensures a consistent mix that lasts longer. Sure, it takes a few extra minutes, but you save hassle in the long run. Have you tried both ways and seen a difference?