If you are going to order your trusses then I would definitely specify that you plan to hang a mezzanine off of it.
From the research I did, a 2x4 has a load bearing capacity of 636 lbs per sq. ft. (note: the "rafters" are 2x6 and the rest of the truss is 2x4) and I don't plan to ever put that kind of weight up there; except for the area over the bathroom walls. Since I've put the mezzanine's up I have not seen any deflection of the trusses at all and I've been up there with more than one person jumping on it and have not seen any issues. I have the wall-side slightly lower than the hanging side (about 1/4-1/2 an inch) so that if anything up there were to want to roll or slide it will head to the wall and not my head as I'm walking by.
Should I notice any issues with it in the future, I will add some posts anchored to the garage floor, but I am trying to avoid that at this time because I like the free-span feel of the floor plan. As I am not a structural engineer and have built everything without plans, I keep an eye on it. I have checked it with a tape measure a couple of times and it hasn't moved since it was built.
If you don't mind having the posts under your mezzanine, I recommend you use them; if you use the same materials that I have.
If you plan to use it for anything more than light storage, I recommend you have your design engineered.
If it's going to act as a second floor then it really needs to be engineered.
Tin:
I lost my in-process tin pics when my old phone crashed. The tin is not under the trusses, it's rolled over them. Basically, when we sheeted the walls the top-most sheet stuck above the trusses about a foot and since we were running the tin horizontally we were able to just curl that foot of tin over the trusses and nail it down, rather than having to cut it off even with the trusses.