With all the conflicting advice in the industry it can be hard to figure out which advice to take and which to leave. If that advice comes from professionals that you respect then it becomes even harder. I've been to a lot of conferences, workshops, and retreats and often the information that comes out of it can be overwhelming. Agents and editors are often looking for different things and they all like slightly different styles. Like you and I, they are individuals.
"Hold the presses Heather! You mean to say the Gatekeepers are human?!"
"Yep, that is precisely what I mean."
I've had agents tell me they didn't like my beginning because it was a prologue. Several have told me don't use prologues, they are awful, no one likes them, people skip them...and all manner of negative feedback on the subject. On the other hand, I've had agents who loved the prologue beginning of my novel and felt it set exactly the right tone. When something like that happens (and there is still no offer) what do you do? There is only one thing you can do. You must stay true to your vision for your novel.
Good advice is that which will improve your novel but not alter it into something unrecognizable. Of course advice on grammar, structure, and spelling is always good. When it comes to character, plot, and story structure that is where things get sticky. A good rule is to follow the advice of those who either love your story idea or are vested in it, such as a signed agent (or one with the potential to sign who has asked for a rewrite), your critique partners, or beta readers (so long as they are the constructive kind).