In flight, Beta range is not something you select rather than something you enter to (unless you specifically move the power levers/throttles/etc below idle).
In flight, in the Twin Otter at least, you enter beta range in EVERY approach as the CSU no longer controls the prop speed, usually around 103KIAS: Flap 10 speed. After this moment, every movement on the power lever you make will see an increase or decrease of prop speed, i.e.: you're in beta range as you are controlling the prop speed with the power lever and not via the prop levers.
Please stop using BETA and REVERSE as if they meant the same: They are NOT the same. BETA range, as V1...Oops said earlier, is the range when you control the prop speed via the power levers instead of the prop levers, which is ALPHA range. REVERSE range is from 0 degrees blade angle all the way to the NEGATIVE mechanical stops, which for the Twin Otter is at -15 degrees. You could say that REVERSE is "located in" the BETA range, but they are not the same.
I haven't got as far as selecting reverse in flight, I've only done up to initial beta or "null idle". Not planning on going to reverse in flight either, but at null idle in flight you get buffeting and a bit of vibration.
As for the taxiing bit, while taxiing you're always at beta range as, again, you are controlling prop speed via the power levers. The prop levers are completely useless on ground for everything other than feathering the prop. Moving the prop levers will see no variation of prop speed, but moving the power levers while taxiing WILL produce a prop speed change (you're in BETA!)
The use of null idle and reverse (i.e. anything below idle stops) while taxiing is a way to control taxi speed and in the turboprops is quite effective.
Beta is not a mode, beta is a RANGE. As a range it has points, between those points it is considered you are in beta range. As it is a range, you can move between the points which make that range, therefore, beta is NOT a fixed angle. The prop actually changes its angle between the range; the prop is in a different angle at the start of the selection of reverse than it is when it is in full reverse.
Hope it helps. It's a concept that it is hard to get your head around of at first.