In my opinion, there are two different approaches to exploiting 12 string guitars. The first is the old style approach of stringing very heavy and tuning low to get a rich, resonant and booming sound. This came about mostly because in the days of the early blues players light gauge strings didn't exist, and the 12 string guitars of the time could not withstand the tension required to keep a 12 string in concert pitch. Musians found that their instruments had a very short life with necks bending, guitar fronts caving in or bridges coming off. So they tuned low, sometimes very low. A fact that was aided by some of the 12 strings at the time having a noticably longer scale length that is the norm today. This is evident in the playing of Willie McTell and Huddie Ledbetter, from the 1940s onwards. Other 12 string players such as Rev. Gary Davis who only used a 12 string in his latter years, would tune only a step or so lower than concert pitch in order to protect the guitar. It is only in recent years that manufacturers have stopped recommending that their 12 string models are tuned half a step or a step below concert pitch. With the availability of light gauge strings, many have stopped doing this. The other more modern approach is to go for a brighter jingle jangle sound using very light gauge strings as can be heard in the playing of Roger McGuinn, George Harrison or John Denver.
I have a slight preference for the old style low tunings, but there are songs that call for a brighter sound, which is why I have two 12 string guitars that are strung and tuned at different ends of the spectrum.
Using a Capo with a 12 string presents it's own set of problems. In general, there only a few on the market that can hold down all 12 strings without muting at least some of them. Re-tuning when the capo is moved is a frequent annoyance. I use a Kyser 12 string capo on my Avalon L32-12 12 string, and never use a capo on the heavy strung Lakewood 12 string.