In the band I am in we pretty much did all our songs in the original key and if we couldn't we just didn't do that song. My fiance was a chorus teacher/vocal coach for 20 years and is very big on getting the key comfortable. She has a wonderful voice and sounds great in any key....I on the other hand have a marginal voice.... this works with a band but in a duo set up I will be more prominent. I have been working on backing tracks and have found I have had to change the key quite a bit to make it comfortable on some songs ie "Bad Moon Rising" originally in D and I moved it to A. I will admit it sounds much better than when I tried to screech to do it in D but how far can you go before you need to make stylistic changes. For me I will have to play the guitar part in a different position so I will need to play it differently. The backing tracks pretty much follow the original.....I just don't want it to sound like a lounge lizard act.