Most modern factory 12-strings are made for concert pitch with light strings. I always thought Martins were made for lowered pitch, but never liked 'em enough tuned either high or low, always preferring Guilds. These days you have Takamines and Yamahas and 'tother Japanese ones that also sound OK at concert.

That said, I much like the boom of a lower tuning, so I use medium strings and never tune to concert (well, that's not accurate -- my cheap Brownsville electric solid-body 12 is tuned to concert pitch, or was before I hung it on the wall and then got so much junk stacked in front of it I can't even get it down to check, alas).

In any case, light strings and modern factory instruments will "take" concert pitch, if you want it.

This is not necessarily true of luthier-made gems like Appolonios or the like. Those you'd better ask the maker before tuning up.

Bob