How is it possible to decide whether or not a choke or balun is
needed, and where to locate it, unless the magnitude of what one MIGHT
wish to prevent is known.

I don't know about wattage, but measuring the
damage that a lack of decoupling does to the antenna
is easy to measure. Quite simple. Try each way,
and note the difference on a stable signal. The ringo
ranger without the lower section lost 3-4 S units
worth of performance in my case, using my line length.
Ditto for any other antenna. I found the decoupling of
simple 1/4 GP's can be improved also. Many, "including me"
often state that once a GP is at say 1/2 wave high, that only
3-4 radials are needed. As far as ground losses, this is true.
But the decoupling from the line can be further improved by
adding even more radials. Going from 4 radials to 8 usually
makes a noticable difference on a receiver...I consider a
receiver as just an poorly calibrated voltage meter in this
case. It's plenty good enough for A/B comparisons.
At HF, I don't worry about feedline radiation too awful much.
If I'm not torching my lips, wanking out my keyer, or causing light
bulbs to flicker in time to my CW or voice, I'm a happy camper.
Not so on VHF/UHF....I consider it critical if you want the best
performance. And....All this was tested in the real world by yours
truly...Over 20 years ago...I've had plenty of time to change my
stance...But I haven't...MK