only use it when the play is right,
If I am totally off the court, maybe like more than 5 feet away from the sideline and it is a ball that was hit with topspin and looped, I sometimes do opt to use the forehand slice, but I find it really risky because a slight mistiming equals a shank. Honestly looping the forehand back deep cross court with good net clearance is probably the best option at the non-professional level. But for pros, it seems that for a lot of fast court players, the forehand slice is probably best option because if their opponent tries to bunt volley it, the underspin goes crazy off their racket many times. Also if they decide to try to approach after your slice bounces, it is also very difficult to control because of the slice put onto the ball.
Now approaching the net is different for me. I try to never approach the net unless I get a ball where I can flatten out the approach shot on my forehand side or slice it on my backhand side because it is pretty tough to flatten shots out with a one-handed backhand, it literally has to be at a perfect height many times. I am really a fan of keeping the ball low when coming into the net because it really hinders my opponent's ability to hit a good topspin lob. So with that being said, I don't think I have ever hit a slice forehand approach. Maybe a flat one a couple times, but never a slice. While yes you can't hit it like a backhand slice, what can be done is by adding massive sidespin as well. Fernando Gonzalez has done this numerous times especially when he has been pulled into the net.
Using it as a change up during a neutral or long rally is something I have never been a fan of. Instead, one of my changeups I use many times is running around a shot that is coming directly towards me and hitting an inside out backhand slice rather. Also the reason why I am not a fan of the forehand slice during rallies is again because it isn't the exact same manner as the backhand slice, both because of mechanics of the grip and probably because of not as much practice of it on the courts. So in turn, playing a quality opponent who can read this a bit early will make him want to sneak in and volley it away.