As a PbP GM myself... not really.

As long as it's readable, logical and does what a post is meant to do aka push the story/conversation along, I'm pretty lax. Several of my players don't have English as native, one is dyslexic and most use phones for posting, so it would be unfair to penalise them. However, I do nudge them towards things like 'get a better spellchecker' or point out a real howler [politely, of course]. I've also gone and manually edited autocorrect epic fails too. [Example: 'your master shows you his sanctum' becoming 'your master shows you his scrotum']

I've had much more issues with posts which have no formatting whatsoever, mass seepage of purple prose, unreadable fonts/colours or posts which seem to make no sense or have utility than spelling/grammar issues alone.

However, I've not yet had a player which from the looks of it couldn't actually write a thing.