Note: This article is most suitable for descriptive grammarians. If you are a prescriptive grammarian, this article may irritate you, and it would behoove you to read this article instead.
Do you always have to follow the rules of "who" and "whom?" Those who have been using the word "who" rather than the word "whom" for their entire lives may be shocked at how often "whom" should be used according to traditional rules of grammar. They may believe that they have been wrong for their whole lives.
The thing these people fail to consider is that the purpose of grammar is clarity. The reason we standardize grammar is so that others can understand us more easily, and if a certain rule prevents us from being clear, then it can be disregarded in some situations. Thus, in informal contexts, using the word "who" at all times may be preferable to following grammar rules. It can make the conversation flow more easily.
Let us be clear: we are absolutely not discouraging use of the word "whom." We love this word and encourage you to use it whenever possible, even in speech! However, we do not want anyone to berate themselves for misusing "who" and "whom." Everyone makes mistakes, and people learn and grow by making them!
In formal contexts, such as published works, following the rules would likely be better. Using "whom" certainly gives your writing a more professional tone. This is why it is important to know the rules; it's not because you must be correct all the time. We would be overwhelmed with felicity if you followed the rules even in informal writing or when speaking, but we are by no means forcing you to. That is the beauty of language — everyone can use it in their own way, and there are multiple ways of getting your meaning across!