I tend to do this not only on forums, but on social websites such as MySpace as well. It's interesting how a higher tier of grammar, spelling, diction, and word choice usually correlate directly to higher intelligence; I'm not saying this is the situation the whole time, just the vast majority. In addition to this, grammar mistakes such as the aforementioned "should of", which may seem trivial to many people, irk me immensely, especially in verbal language.