I'd have to go check (it's not here where I post from), but I seem to recall the owner's manual that came with my '96 D-25 said it was ok to tune 'em (12-ers) to standard due to their construction, but by then the only strings they offered were the lights which that early manual says is ok, anyway.
Apologies for veering back on topic but I did check my '96 D25's owner's manual last night.
It also had a dedicated page on 12-string tuning and while it didn't say that they were built to handle standard tuning, it
did suggest using
light gauge (B1240 or B1250) strings due to the high string tension of 12-strings, and then showed how to tune to C (EADGBE) (whew, vindicated on that point, at least), with a note that one could also opt to tune 1/2 step lower and use a capo.
I think the manual itself was a bit out of date since at the time the standard 12-string set was
L1250, .010-.049 pb's.
Not sure what the B1240 was, I'm guessing it was even lighter than the B1250 since it shows up between the B1200 Silk'n'Steel's and the B1250's on '87 and '88 price lists