letter - I think that happens anyway.
I don't type how I talk, the odd word slips through, but...
I don't ever say yes, I say aye and I tend to type yeah or yes instead, words like wee I use sparingly where I say them all the time, but a wee puddle for instance does not read well if you don't know someone is Scottish, lol and I wouldn't use words like juice how I normally would.
I end up using things like SEN and SN where I just wouldn't ever ever say them, not just because it's a different term in Scottish education. If a pupil has additional support needs that's what I'd say as that's what it is called up here, if I was talking about someone with a disability then that is how I would put it. If for instance I was discussing in conversation things like autism...I would always call it a disability and only use SN in here if I'm replying to someone else.
Then of course there are things like gonnae and dinnae, didny and wasny that I say but wouldn't ever write in any format anywhere, lol.
When it comes to spelling, only things like loose instead of lose bother me...and that's just because it's a different word and I have to reread it before the sentence makes sense. If I can read it, I don't care.