Having just started flying light aircraft again after many years just flying jets for a living, I find this quite interesting. However, my take on the POH is a little different.
Since the days of product liability suits in the US, POHs have been quite restrictive and are more of an exercise in limiting liability to the manufacturer, than a helpful document to the pilot. There is an awful lot that isn't in there, because the manufacturer wants to limit risk and would prefer that you only ever flew straight and level, and took off and landed from large concrete runways on sunny days with no wind. Therefore, the true capabilities of many aircraft are never properly laid out in the POH.
I would take the view that you should never do anything that is specifically prohibited in the POH, but by the same token you are free to do anything that isn't specifically prohibited, as long as you apply prudence and good airmanship. I don't agree that this makes you a test pilot - that is a standard saying in GA that has little basis in fact, and is just used by instructors to scare students into behaving themselves. Most light aircraft are absolutely benign unless deliberately provoked by serious mishandling. For example, if you were to ask a group of ten bush pilots about takeoff and landing techniques, you would probably get ten different answers - all valid and all of which work for the person concerned (and probably none of them in the POH). Some topics, like mixture control/leaning, have been endlessly argued by proponents of many different methods, none of them specifically approved by the manufacturer but all of which work.
So my point is... use common sense, take the POH as a starting point, and develop your own techniques. Don't do anything specifically prohibited by the POH, but by the same token don't not do stuff that is sensible, just because it isn't written down. Most of what you need to know in aviation isn't written down!
And on the subject of takeoff technique, you can take off with any flap setting you like... you will always get airborne... but some settings are sensible and some aren't. I once accidentally took off in a 172 with 40 degrees of flap after the electric flaps failed during a touch and go... we got airborne OK (with virtually no ground roll!), but if we had lost the engine, we would have been dead meat.
And just to illustrate that point a little more, the BAe146 that I used to fly could take off with flaps at 18, 24 or 30 degrees. Full flap was 33 degrees, so a 30 flap takeoff was virtually a full-flap takeoff. In terms of performance, when taking off from a short field (around 1200m), Flaps 30 was mandatory. Went up like an elevator!
Use common sense!