I'll have a go at answering this.
I think it really depends on the judges opinion in the end. I try and keep images within the boundaries of either realistic and/or 'the image I wanted to portray' / what I imagined the shot looking like before I took it.
However, I throw rules out the window a little when it comes to the creative category.
For example, this wasn't an actual entry, just part of my 52 (that I never finished), but I don't think it's over processed for a creative category (The theme was Old)
The original looked like this:
This is obviously not following any rules at all...
A Small World by
rowee2, on Flickr
Now if you did something similar in the Landscape category, it may well be a going too far. But actually the picture is very similar, just distressed (The Nintendo shot)
For realism, I
like to try and keep my processing to simply enhancing the original image, or trying to make it more like how it looked to me at the time if the camera hasn't coped with the light. I say try because sometimes, heavier editing just makes the result much better (Or you just got it all wrong in camera).
The picture below was from a couple of years ago, I think the processing is minimal but necessary:
original by
rowee2, on Flickr
edit by
rowee2, on Flickr
Ideally, if I could have lightened the dark tree in the background, I would have, but the information just wan't there unfortunately.
Often a competition will have rules or guidelines regarding processing, obviously if that's the case, you'd do well to follow them. If not, I guess it's down to your best judgment. Personally, glaring and unrealistically vibrant colours in an image is a turn off, but for others it may be a winner.
As for adding and removing, I think adding is a step too far (just my opinion), but if you have a beautiful landscape, and in processing you notice a Coke can (other soft drinks are available) that's been chucked on the floor, then removing it from the image is absolutely fine in my book, and I'd be surprised if anyone had a problem with that sort of thing.
And actually, while typing this I just thought of another picture I took recently that has had things 'removed'. Again not a competition shot, more of a holiday snap, and the cloning isn't amazing either but I think this would be acceptable editing of an image...?
original by
rowee2, on Flickr
61 by
rowee2, on Flickr
Not a pro, and don't enter competitions all that often, but that's kinda how I see things.