I do when teaching people to cast, or when I am trying to make casts in areas where I want my backcast to go in between trees or somthing crazy. its a good thing to do if you think there is somthing wrong, but when I am actually fishing it is very rare for me to do anything but look where I am casting. also, timing is important but I think it is truly more about feel, the timing changes with every foot of line out of the rod tip, and every bit of wind, and every size of fly, looking at you backcast will overtime help you understand what you are feeling.
spey casts are great, no backcast required
I did a lot of cross roll casting this weekend due to the situation I was in not able to wade . So RK had explained the basics but every conditions will make you change your casting in order to get your flies to where you want it to. Spey casting is fun for sure!
I always watch my backcast when casting the first few times in a new spot on a bank or near a bank. Seems like whenever I assume I don't need to worry about what's behind me is when I snag up. But if I am casting "in stream", I don't usually watch it.
If I need to add a lot of line to a quick single cast (like when not going to false cast and I see a fish rise 10 feet past my current target and want to cast to it without moving), I'll watch it due to the big change in my timing and rythm.
I also start watching my backcast again as soon as I feel something get a bit wonky with my casting. The problems are almost always more evident in my backcast.
I only watch or glance back when casting in tight quarters, like yesterday on a small WV stream that was choked with branches. There were places where even a roll cast seemed impossible. That's where I resorted to the "slingshot" method of holding your fly by the hook bend, pulling back to flex the rod a bit, let go, and punch the fly in where you need it. I caught a few small brookies using that cast. It's close range only, but in a few situations yesterday it got me fish. When trying to back cast in cover, I don't typically watch the back cast, but I will glance back prior to the cast to see if I need to hold the rod vertical or cast sidearmed and low to the water.