But it's always worked just fine -
This comment may be true for some, however it could be just a matter of time until an incorrect electrical wiring project could turn into a very bad result.
The #1 Mistake with anything is the failure to read and understand instructions, followed next by the choice to not do the project correctly. This creates what could be the start of a very dangerous situation.
Do it right the first time is always my advise, this is why performing all work with a permit and having the work inspected is essential for all electrical projects.