There is, essentially, always a raging debate on somewhere in Photography about whether or not, and what types, of manipulation are permissible. I think I have been pretty consistently on the record as thinking it's all idiotic and arbitrary.
Today I feel like writing a little more about it.
Let''s start from the basic premise that photography is interesting specifically because it is a literal tracing of reality, through the mechanism of optics and some sort of sensor. It is a literal record, subject to some limitations, of what's in front of the lens during the time of exposure. This is what makes photography Not Painting and Not Anything Else. It is what makes everything else Not Photography.
A photograph isn't reality, and in truth it's an inaccurate representation of reality, starting from the fact that you've thrown a frame around something, and proceeding through various technical limitations of the medium, and onwards to various manipulations to the underlying latent image (be it made up of exposed silver halide crystals or a digital file), to the final rendition bangin' on the retina of a viewer.
Still, it is the connection to reality that separates a photo from a painting, from a drawing, an etching, what have you. It is that connection that is the essential "photograph-ness" you're working with.
The key point here is that everything that you do, starting with putting a frame around something in front of the camera, and finishing with the choice of paper you print on, takes away from that reality. The picture's connection to reality is weakened at every step.
Some of these steps are mandatory. Without a frame, without a lens, without some choices for the final rendering, these is literally no photograph. Some of them are less mandatory, you can make more or less radical changes to tonal placement or to color rendering, or what have you. You might choose to remove elements or paste elements in to the final picture.
All these things are done in order to make a picture that looks the way you want it to look, that expresses what you want to express. So, you're explicitly trading that which makes a photo a photo (connection to reality) for expressiveness.
And that's OK. That's the name of the game, in fact, it's what photography is all about.
As with all systems in which you are trading one thing off for another, though, one needs to take care. This is why one shouldn't manipulate wildly. You are, in a sense, spending "reality" with your changes, and usually more of it than you think you are. Squandering this finite resource for trivial reasons is silly, and a waste.
Make your sacrifices count.