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Upvaluing globals is a micro-optimization that you shouldn't bother actively thinking about when you're coding; looking up a variable is very fast and any addon that would see an appreciable difference from storing a local copy to access is probably not going to be saved by this.
Write your program to do what you want it to do, then if something is impacting performance, clean it up. Anything that isn't an obvious performance gain should be deferred. Premature optimization can introduce unexpected behavior that can end up wasting a lot of your time tracking down and debugging.
Creating a local copy of a variable comes with the unintended (or intentional) consequence of preventing it from being replaced or hooked by other addons later in the loading process.
For example, let's say you want to monitor when another addon modifies a cvar so you know not to touch that cvar in the future, so you hook the global function SetCVar. When an addon calls SetCVar it also runs your function and everyone is happy.
But what if this addon creates a local reference to SetCVar before you hook it? Now when it calls SetCVar your function hook doesn't run, and everyone is sad because your addon ends up overwriting their addon's settings because it didn't see the change it made.
Additionally, lua actually has an internal limit of 60 upvalues. This isn't something you normally have to worry about, but you could conceivably run into it by redeclaring a large number of globals as local variables in your addon, and then trying to access them all in a function.
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