You certainly don't have to cap...
providing it is sufficiently protected from any known risks of mechanical damage..
e.g. Impact 522.6, Other mechanical stresses 522.8
AND
it is vertically or horizontally in line with an accessory, (safe zones)
Which in your average domestic property whilst the wall is waiting to be covered, the only probably impact is the Plasterers or other builders trades...
e.g.
on the same wall if it was already plastered, you could clip your cable on the surface...
or along the top of the skirting.. if you were that way inclined.
and all be perfectly compliant.
And thats probably more susceptible to impact?
p.s.
I personally DO cap ... just think it looks neater...
stops the cable bowing out from the wall...
probably less nails to bang in?
especially if 2 or more cables coming down the wall!