One does ask for left/right around anything big sticking up, but to do that one needs to be in VMC in the first place.
That's where the discussion of cloud top determination comes from. Let's say you are doing an IFR flight from the UK to Poland; that's some 700-800nm and European airway MEAs, not to mention permitted routings, will put you either in IMC or above it. The option of going below cloud is rarely available.
So one needs an idea of the tops. Let's say they are forecast at FL120. You flight plan it for FL140, with an option to climb substantially higher (ATC will always let you climb "due weather" or even without any reason, in the FL100-FL180 range, IME).
Then, as you are zooming along in the sunshine, you can do avoidance of CBs etc etc as you suggest.
Airliners and well equipped other planes can do this from inside IMC too, using radar, but then airframe
icing comes into it. European airway routes will likely take you into 0C or below, even in the summer, so you'd better be in VMC for most of the enroute section.
One can plan IFR flights around the bottom of the airways, say FL070, but one is very likely to just sit in cloud there, and I would personally reserve this option purely for descending below the 0C level, having been picking up too much ice. You've got to make sure there is no terrain close to the 0C level
The above is it in a nutshell.