For me a lot depends on how much time I have to set up for the ball.
If I'm given a deep, slow, topspin shot I will use my hand as a guide. I don't like physically point straight at the ball as it's coming at me, but instead, I use it more as a means of loading my shoulders/upper body for the shot. Actually, my non-dominant hand has more of a general guiding/sweeping motion sort of directed toward the incoming ball. My hand never stays in one spot, though, it is moving with sort a motion that basically starts my upper body rotation for hitting the ball.
Now if I have no time, lots of things are different. Sometimes I have zero chance to get that hand out on, say, maybe a REALLY deep topspin return from my opponent. In this instance, I am really only looking for a defensive shot, which is easily produced without said hand motion.
So I guess my general formula would be: When I have time to go for an aggressive fh, I will do a well timed guiding motion with my hand to effectively line up for ripping a huge shot. But, when off balance or short on time, I frequently don't have the opportunity to get that hand up, and therefore cannot go for anything more than something defensive.