A rule of thumb is not exactly a rule, it is more like a good idea. Example: "I don't invite people to my home unless I know them well, as a rule of thumb."

You don't have to follow a rule of thumb because it is just a good suggestion. Example: "Do you always have to use white paint?" Reply: "No, that's just a rule of thumb. You can use whatever color you like."

If you have a better idea then you can do it your own way but it usually safe to follow a rule of thumb because it has worked well for other people."