Norrissey,
Many people use alternate tunings on their 12s, especially drop D (strings 11/12 down a full step) or DADGAD... But standard tuning is fine! If you're going to do that, use light or extra-light gauge strings -- no mediums or heavies(well, that's my advice anyway).
I use standard tuning but tune the whole guitar down 1/2-step and apply a capo on the first fret if I need to be in normal pitch. Some of my guitars are down a whole-step.
The only Guild 12er that wasn't designed for standard tuning (at standard pitch, that is), was the unobtanium F-612, which has a long scale (26.5", I think) and huge jumbo body (18" at lower bout). This guitar should not be tuned to pitch. John Denver had a couple of these and any time you see him use it he's playing drop-D tuning with a capo on either 1st or 2nd fret (i.e., pitched down 1/2 or whole-step). Every other Guild 12er that I'm aware of is at a normal scale length and should be fine at standard pitch.
As an aside, as you embark on your quest to find a great 12, the neck angle and the fretting of a 12 is particularly important. Thing is, with 12-strings, if the guitar needs work, you will never play it because it's uncomfortable to do so. A properly set-up 12-string is a joy to play and really isn't noticeably harder to play than a 6-string.