My own view is if someone's submitted a track to your song then as the producer you should pretty much have carte blanche to do whatever you need to with it to best serve the song. Production has changed with the advent of the DAW , there's an awful lot you can do either in a corrective or creative way.
As the producer - writer, I would agree. If it is your song, then you have the vision of that song in your head.
But if you are the mix engineer it is different. If it is someone else's work, you are making an educated guess at what is best.
Things I do without permission.
Tune and edit BGV's
Adjust track timing or just a note or two
Tune lead vox if they don't need to much work.
Cut and paste a repeated region where it was done well in one part and missed in another.
Compress, EQ, pan and FX to my taste
Cut parts to make room in a busy mix.
Things I ask permission for:
Cutting up and rearranging a solo
Hard tuning a vocal
Cutting up drum performances
Supplementing drum-tracks with samples
Looping drums or bass
Moving a solo
These are just some examples. But a PM is always appreciated when altering a track. Of course I would be remiss if I said I haven't done things without permission.
Nick