You’re right, this is one of the ways to do it. It would only not work and throw errors if your “String” variable is not a string.
Personally, i usually prefer to use something like..
myString = string.lower(myString)
But its really the same as doing
myString = myString:lower()
assuming that myString is actually a string, however.
The “long” version has one advantage, it actually works if myString is a number, while the second one errors in that case.