Anti seize is your friend. Having that little bottle and using it will save you, and the next owner of your car much aggravation.
I would never change any 'bolt' size fasteners with stainless unless I was really sure of the strength. Stainless bolts are marked grade 4 in the US[usually] and while I view them as an upgrade from a grade 2 regular bolt, only just. Standard bolts and never seize are your best bet there.
304 stainless is gummy stuff, and if you try to put much torque through it the threads can move and, yeah, gall up.
On my car I think the small M4 screws are threaded into little tinnerman style edge clips.
This is what galvanizers think about stainless and zinc:
I tend to replace tiny fasteners that you don't take out much with stainless, because the next time you see them the head will be rusted away.
you are probably not driving in the winter in salt, and are probably taking your aero tray off for every oil change, so I doubt the steel bolts will rot away on you, so dealers choice there. I use stainless because I hate the look of rusty hardware. The next time a stainless fastener causes some kind of corrosion on my vehicle will be the first time, others experience may differ.