I always thought the following saying was my bible in terms of how much i can edit a photo....
"A photographer uses lies to paint the truth"
No idea where i heard it, wasnt so long ago in fact but it stuck.
I will edit a image to make it as close to how i saw it with my eyes, there are obvious exceptions such as some more artistic renditions i have done such as HDR etc however i try stick to it as a general rule!
Personally I feel that even HDR is closer to photography than this is. With HDR you're still using your own photography to create an image... the scene you see in front of you is still being shown, just tweaked to make it seem surreal. With this method, you're taking what is essentially another person's photo and sticking yours on top. I feel that it crosses the line.