As a youth in the 1970's we got marked down at school for incorrect spelling, not just in English, but in every subject. I just always thought that it was important to get it right. As a parent in the late 1990's I found out at "parents evening" at school that the teachers really didn't seem to attach much importance to spelling, grammar or punctuation and my children reminded me frequently that as long as the teachers could understand what they had written it was fine. I strongly disagreed and made my view clear to my daughters and their teachers.
My offspring are now in their 20's and they always ask me to proof-read important documents like job applications, because, as they say, they want to make a good impression.
I always reply (before proof reading it!), "Nah, it'll be fine, innit, vey can understand it, spelling an all that stuff don't matter".