When I first started playing Starcraft 2 multiplayer with Terran, I really didn’t use Mules. This was a big mistake. Mules can give you a big advantage with all the extra minerals they can produce.
Then, even after I started using Mules, I wasn’t doing it correctly. I would drop them next to the minerals and then tell them to gather minerals from there. This wasted a few seconds that they could have been gathering with.
What you should be doing is dropping them right on the minerals. That way when they spawn, they will start gathering immediately.
Also, keep in mind that if you have say 3 command centers (all with Orbital Command), you can cast down mules from each. What I do is hot key my command centers and then call down mules on my new expansions. I can quickly mine up all the minerals from the new location and then I don’t necessarily have to worry about losing 12+ scvs.
So if you play Terran, then use mules. They are probably the best part of Orbital Command because you can get 300 minerals out of them before they burn up.
The Shokz guide talks a lot more about how to use mules more effectively.

