I think this is definitely something to bear in mind:
I recognise [that] different modes of communication demand different voices. The way I write a story for Lifehacker is quite distinct from how I express myself on Twitter or in a text message to friends. The vocabulary, tone and structure will all vary. But one key element won’t.
In all those contexts, I use capital letters and punctuation correctly, and aim for accurate spelling. I don’t say to myself “this doesn’t matter”. It always matters. To gain the habit of accuracy, you need to aim for accuracy all the time.
I think that when it comes time to write that perfect introduction, query or cover letter, you will thank your deeply-ingrained habits.