The rule of thirds is very important for composition, it can sometimes make or brake a photo. However, I can often tell where the grid lines are, I’ve gotten used to it now. If I’m taking wedding or event photos, I’ll put them on though.
Sometimes I do, sometimes I don’t. Rules are meant to be broken, this applies to every discipline. Of course the photographs turn out horrible sometimes because of this, but I managed to get some unique shots as well.