The line is quite easy to see actually. As long as you don't lie to yourself and your intended audience/client, you're well within the line, regardless of the amount of digital manipulation done or lack there of.
At one end of the spectrum, we have portrait/fashion/advertisement photography where pretty much anything goes, as long as the client agrees to the end result. And then we have reportage/sports photography, a genre where the intended audience, the public, wants to see what they could have witnessed with their own eyes had they been there.
For instance, if a PJ shooting an aftermath of a forest fire adds a few adjustment layers in PS to darken the half-burnt trees to give a high contrast charred effect, he/she is not capturing the scene as it would be seen if the public themselves were there, but as he/she wants to see it. Art vs reality, work ethic over personal desire, whatever you call it, it is an obligation that comes with the job to think straight.