This is all great info, glad to hear so much about the 12th house. Regarding house systems, why is there more than one system anyway - surely just choosing whichever one you like the best isn't the way to go here... is it?
And I wondered from reading all the fascinating stuff posted here about the 12th - being widely an unfortunate area, hidden enemies etc. How then to interpret having my North Node on the 12th cusp? And in Sagittarius, this seems like a contradiction.
There is a really big history of house systems.
Basically, a house system divides up the heavens on a diurnal basis in terms of both space (distance) and time. Different house systems were devised to find better ways of doing this, often as an exercise in spherical geometry relative to the movement of the sun in a geocentric model of the solar system. Some good discussion of the geometric problem are in:
http://www.scribd.com/doc/6495552/An-Astrological-House-Formulary#scribd
The origin of the idea of houses probably goes back to ancient Egypt, with the mythical journey of their sun god Ra. We do not truly know how they originated but that's a good guess. (See Deborah Houlding,
Houses: Temples of the Sky.) Their system related to the heavens as their clock and calendar. Ra's journey proceeded in stages called temples, some of which seem to correspond to the Hellenistic houses. Alexandria, Egypt, with its blend of cultures, was a major center of ancient astrology.
Overlaid on this symbolic solar Egyptian "clock" were Greek thoughts about number symbolism and geometry.
With the passage of time, various astrologers worked on improving the sensitivity of house cusps, notably for timing events. The whole sign house system has its proponents and advantages, but using house cusps to time events when the cusps are all at 0 degrees isn't one of them.
I'm afraid there isn't a better answer as to "which one is best"-- despite many astrologers' strong assertions to the contrary-- than which one
works best for you-- or for your chart native. Regiomontanus is the traditionally preferred system in horary astrology. Selecting your house system will take time and a lot of practice. If you do not read a lot of charts, you will not develop sufficient facility to make an informed judgement about your preferred house system.
Michael Munkasey, linked above, said it best. The "best" house system for a particular person, question, or event, is the one that best describes the situation under examination. If a situation is best described by the moon in the 8th house, then you don't want to choose a house system that shifts the moon into the 7th or 9th house.
Each planet, sign, and house has multiple potential rulerships that are consistent with their core meanings. To know the meaning of "the north node in Sagittarius in the 12th house," for example, you have to read the chart as a whole. Then you can get a better sense of whether the 12th is likely to mean volunteering in a hospital, solo meditation, or growing personally through coming to terms with secret enemies-- or possibly one's unwanted tendency to be deceptive. Is Jupiter the planet ruling the 12th house cusp? If so, what is its situation? If Jupiter is in the 9th house of religion, then possibly solo meditation would be a good meaning. If Jupiter is in the 6th house of illness/health, then maybe volunteering in a hospital.
Remember, a sign says "how or in what manner" a planet or point operates. It is the "how" not the "what" (NN) or "where" (house). Sagittarius is the mutable fire sign, by nature adaptable and initiative-taking or enthusiastic.
If you find your NN moves into the 11th house with different house systems, see which house seems right to you as you learn more about the houses. Maybe your growth lies in your friendships, for example. Recall that the moon's nodes always move in a retrograde direction, so if it is on the cusp, it is working its way backwards.
In choice-centered astrology, you also have some choice as to what you want your NN to mean-- within the ground rules set by its placement and the entire horoscope.