I've been drymounting my prints onto foam core and haven't had any problems with flatness. I attach my 4-ply matt over that with 3/4" ATG or "snot" tape because it keeps the matt flat against the print without any spaces in between. I've always been dissapointed when viewing someone's great print that has a warped matt attached only by hinged tape at the top. It may be the one of the most accepted "archival" methods but it often looks terrible and there's no guarentee that it won't warp once it's delivered to a different climate.