No, it's not irrelevant. There are just a couple gray areas but plenty where the rules are completely clear - such as with plurals. Writing "arena's" when you mean "arenas" makes your writing quite awkward to read for somebody who understands the rules of grammar.
Mr. Tennis, whether or not grammatical rules exist is not a matter of opinion. Whether or not they are important is, but even you must have your limits? (Not that there's anything 'wrong' with your grammar, other than the lack of any capitals, which I personally don't find distracting.)