Having recently gone back through some of the logs I've seen some of the horrible mistakes I have committed in terms of spelling and grammar; some of these were done while suffering an eye-infection so the inability to clearly see what I was writing or reading contributed to those; others thought, well, I tend to try to type fast and get a response in and know that this causes some pretty bad mistakes to happen. Some I do catch and others not so much.
At the end of the day, I typically don't pay too much attention to spelling and grammar during RP in EVE. The grammar doesn't bother me as that is more a writing convention; the typical verbal conversation between two people rarely adheres to rules of grammar. And well, we don't see the letters of the words we speak. On the other hand, if I have to spend five minutes trying to decipher what was written and the point trying to be made, that becomes problematic.
As a rule of thumb, I don't call out spelling or grammatical mistakes made in IC interactions; I might call attention to a mistake like referring to another individual by the wrong gender or other 'slips of the tongue'. I've had some interesting interactions by actually misinterpreting a statement.