I did some research and according to IMDB
X-Men aspect ratio - 2.35 : 1
X2 X-Men United aspect ratio - 2.35 : 1 (Cinematographic Process Super 35)
X-Men The Last Stand aspect ratio - 2.35 : 1
Mars Attacks aspect ratio - 2.35 : 1
Austin Powers Goldmember aspect ratio - 2.35 : 1 (Cinematographic Process Super 35)
Hope it helps from this information I would assume that the only ones that could be open matte would be X-Men 1 and Austin Powers