a good teacher (of anything, not just music) knows that they can learn something from their students, and that it's not a purely one-way street. bad teachers are threatened when a student knows something that they didn't, but good ones see the opportunity to improve.
regarding fingerings, the "standard practices," whatever those are, are generally designed to allow the most comfortable execution of a passage, but they may not feel that way at first. over time however we often learn that they are indeed the most effective. as you mentioned, many of the fingerings listed on scores are those of an editor, and none of them should be seen as rigid. I'm sure rachmaninoff used fingerings that those of us with normal-sized hands can't even contemplate, but that shouldn't stop us from playing his music. beethoven often wrote fingerings that are terribly awkward, and much as I want to respect his music, I don't see that as an affront as long as the right note is played. but, in contrast, with many etudes, the fingering is the element being studied. for a simple example, the hanon thumb-passing exercises: no sense doing them if you're not passing the thumb.
in general, if I see a fingering listed on a score, I try it, because it saves me the time of trying to figure it out myself. but I don't hesitate to substitute something that works better if I am struggling.
regarding repeats, some repeats are more integral to the score than others. the repeated exposition of a sonata, as mentioned by iroveashe, is often unneeded, but sometimes it is required to give balance, especially if there is a lengthy development to follow or if the theme wasn't stated very strongly initially. repeats within a section are generally necessary, such as those within the second movement of beethoven's appasionata (op 57). to paraphrase animal farm, "all repeats are equal, some are just more equal than others."