I have a cynical feeling it might be up to the theaters themselves to respect whether it's in Atmos or not. And this is in a time when a lot of theaters with both 2.35 and 1.85 (or I grudgingly suspect, 1.78) screens can't be bothered to care whether they pick a screen that is properly optimized for [whatever movie in question].
In my experience, if a theater regularly participates with Fathom, it will usually be the same screen every time. For instance if it's Sam's Town I can already tell you the Fathom event will be in auditorium 13 (except this year TWOK is getting shafted with auditorium 9 which is about half the size).
Only real answer is to know your theaters. And know your screens. Which is the Atmos one? Which is the chain-patented specialty one (DFX/JBX/whatever)? Which ones are "optimized" at widescreen versus standard? Which ones are maximum versus minimum occupancy? Keep a list:
AMC Town Square 18
Constant image width (“IMAX”) - auditorium 2 (luxery, 12 rows 2+10x22)
Constant image width (Dolby ATMOS)- auditoriums 1 (luxury, 10 rows 3+7x22)
Constant image height - auditoriums 3-4 (X-large, 18 rows 5+9+4x25/29, occ 469)
Constant image height - auditorium 5-7 (large, 16 rows 5+11x17/21 occ 296)
Constant image height - auditorium 18 (medium, 14 rows 4+10x17/21 occ 260)
Constant image width - auditorium 17 (medium, 11 rows 4+7x13 occ 144)
Constant image width - auditoriums 8, 12-13,14 (small, 11 rows 4+7x11, occ 144)
Constant image width - auditorium 16 (small, 9 rows 3+6x13, occ 118)
Constant image width - auditorium 9 (small, 9 rows 3+6x11, occ 118)
Constant image width - auditoriums 10-11, 15 (small, 8 rows 3+5x11, occ 105)
