Yes, they do. I've had a number of characters either forge or deal with potentially forged letters, and I do note when someone's writing style changes. It's also one of the tell-tale signs for multis if they all have the same writing style, so I keep an eye out for that as well (though the better multis disguise that).
I agree fully. It's even more so obvious for non-native English speakers. They consistently write the same way and always make the same small mistakes, etc. It's pretty easy to identify them.
