Reversed polarity lnbf's are used on a T90 (Torodial) dish for getting the Circular (DBS) satellites. A T90 has 2 reflectors and gives a mirror effect for the Circular satellites, therefore you need a reversed lnbf to get them.......
which btw an fta ird doesnt care about. you can use a regular circular lnb on a T90, just your R tp's scan in as L and your L tp's scan in as R. no biggie.
As far as I know, there are only 4 satellites that you could view that would even be circular to begin with. As the others have said, you would not need to worry with reversed polarity unless you buy a T90. Some sellers of the T90 even include 3 reversed polarity LNBF's with a T90. Back to my original point, the 4 satellites would be the two Bell birds and two of Dish's birds. You will only find NASA on Dish Net along with some Dish FYI channels and I think there is 1 channel that says something about a PPV event that is going to be on a certain channel, but, without a sub, you would never see that channel anyway! As far as the Bell satellites I think they only have FYI channels if those. All the free music channels that used to be on them are gone now. I wish I could find some free Cricket on Bell, that would be sweet. It's on my DTV sub, but, I don't subscribe to that package as I don't like it THAT well because the upcoming package that runs from April to whenever is $60!
NASA might be worth an LNB position on a T90 if you are into space stuff, but, I can get that myself on DTV so, I don't plan to get NASA on my T90 that I am building.