Hi ricx,
Welcome to T2W.
I don't trade FX so, according to your stringent requirements, my comments won't help you or warrant your respect. That's fine by me! Nonetheless, IMO you are asking the right question to the wrong people. YOU should be answering your own question based on your own research and evaluation. Even if I was an FX god that made squillions each day, it shouldn't affect where you place your S 'n' R lines. Besides which, some things are cast in stone and don't change much because they are fundamental to the way the market functions, i.e. the theory.
1. Time Frame
This is own to the individual trader. S 'n' R levels used by a day trader scalping a 1 minute chart are unlikely to be of much use or interest to a swing trader holding positions for days at a time. The exception to this might be where there's a confluence of S 'n' R across a number of timeframes.
2. The Theory
Support levels are points where selling pressure dries up and demand outstrips supply. Price finds support at the point where there are no more sellers prepared to sell at a lower price. Buyers are then sucked into the market as price starts to rise. Resistance levels are points where buying pressure dries up and supply outstrips demand. Price finds resistance at the point where there are no more buyers prepared to buy at a higher price. sellers are then sucked into the market as price starts to fall.
3. Importance of levels
The important ones are the ones where institutional stops are in place and are likely to cause a reversal. If you can work that one out, then you can piggy back a ride with the big cats and your life will be easy as you're trading with the money flow rather than against it. Have I sussed out how to do that with a high probability of success? No, sadly not, but I'm working on it!
4. Anything else . . .
The basic part of your request I've answered, albeit superficially. As 'jiggly' advises, do a Google search as there's mountains of info' out there on this very subject. As for the Holy Grail part of your request - i.e. 3. above, don't expect anyone to give you this kind of info' for free. You'll either have to work it out for yourself or dig deep into your pockets and pay for it.
Good luck.
Tim.