I think really what this comes down to is whether you want to get your back up about it and have a confrontation or not. I am with EllenS, Toots, and Lynn2000 -- usually the "must" really means "This works really well/looked beautiful in my experience." So you can take it as that and just listen, ask them about their experience, or say "We'll have to look into that!" or even "Great! We'll do that!" (and then do as you please, of course).
In fact, if you answer, "Oh, no, we do not 'have to' do X," I bet the person would say, "Well, come on, I didn't mean you literally have to, I'm just telling you it worked really great at Petunia's wedding" and so forth.
"Have to" and "must" are, I agree, unfortunate, inartful phrasing. But that's all. There is no need to interpret them as commands unless you are spoiling for a fight.
I'm talking here about the kind of details that have been discussed in this string -- numbers of bridesmaids, programs, flowers, etc. -- not things that have deep emotional or religious loads, like seating enemies separately, wearing appropriate clothing for a house of worship, excluding a close relative from a large wedding, and of course thank you notes! -- or reasonable requests from parents, especially if they are hosting -- I'm also not talking about etiquette "musts" such as "You must invite both spouses" and "You must treat your guests equally; no B lists."
When someone tells you that you "must" have an ice sculpture or monogrammed napkins, though, ask yourself if what you are really worrying about is having your wedding your way, or winning an unnecessary argument and proving no one can tell you what you "have to" do.