Critical. Always, critical.
Just because others have lowered standards is not an excuse for the writer to lower their standards in things that are intended for others to read.
I was in a zoo this past weekend--great zoo, wonderful place, they treat the animals well--but the exhibit labels were full of typos, comma misuses, and grammatical errors. Which minor-league bugged me, until I saw a little kid sounding out the words. How was the kid supposed to learn the right ways if the wrong ways are posted in official places?
For personal writing--for blogs, for diaries, for first drafts and stuff no one's supposed to see--neither grammar nor spelling matter very much. For things intended for publication, why would a writer send out something other than their best work?