This is a very important and a relevant factor in my experience. Being headstrong is not bad in this case.
If your SA doesn't have a good relationship with the SM and your SA's convincing power is not great, more often than not, the item that you like will be sold to someone else..it will go to an SA who lobbies and advocates for her clients the best. One time, I asked for a QB. SA said: "I talked to the Manager. There is none (If I am right, I interpreted this as all the bags that they have have already been pre-assigned to specific clients so none was available in the specs that I liked at that moment). But I see one coming in in these specifications. I don't know when exactly it is coming in. If you're interested, I'll put down your name for it and when it comes, it's yours." I liked the specs and said yes. But she knows that it was for a specific occasion. I resigned to the fact that I have to wait for a while. That same evening, she texts again saying she found one magically and told me the specs. I loved the specs. I hurry to the store the next day to purchase it. Thanked her profusely. She said something like: You are welcome. You just don't know what I had to go thru to snag that one for you. Loyalty pays off in my book. That's very important to me and is rewarded!
So if you are starting to develop a business relationship with your SA, make sure that her voice is something her SM/Director actually hears.