On BBC Breakfast this morning they were talking about punctuation, spelling, and grammar, and asking the question, 'Does it matter?'
Yes. It does matter, because it forms the basis of people's impression of you.
It's the same as going out: are you just popping out to the shop ten minutes before they close to get some milk? Then go in your track suit bottoms, trainers, and manky hair, by all means.
The writing equivalent goes like this:
Are you just scribbling a note or email to a very familiar friend to say you've gone to get the milk, or similar?
Then misspell away. The tea needs making. Go! Rush! Get the milk.
But. If you are going out to a fancy party, or a job interview, or just out into town, you need to think more carefully about your appearance.
Same with your writing. Think more carefully, if it will betray a negative impression of you.
It's the same as going out with your flies open, or your skirt tucked in your knickers. You just wouldn't go out without looking in the mirror.
If your writing is full of mistakes, people will think you are lazy, uneducated, disinterested, and you don't know what you're talking about. Or worse.
However: don't get so fussy about punctuation and grammar and stuff that you miss out on the genuine-ness and heart and emotion of what someone is offering you with their writing. Would you chuck an apology back in the writer's face, saying 'You cretin, don't you know that when it + s has no apostrophe when it's possessive?!'
By the way: I was the one who went to Waitrose in the night for milk, after travelling for 36 hours. Turns out this poshest of supermarkets was having an after-hours card-holders' soiree, with wine tasting, flower arranging, fish monger demonstrations, the lot. I was too embarrassed to stay and get the milk.
And this: learn your rules. Then you know absolutely whether you are right or wrong, you learn a great deal about playing the language, and when someone tries to correct you, you can get your little copy of Fowler's out and turn it into a curious pursuit to find out more, together.