[QUOTE=RugBug;7132457]
IME, picking it up “when it is needed” means you are too late to correct whatever problem you are facing. You MUST have it with you at all times to use immediately when the problems arises. [/QUOTE]
You would have to know exactly when it was being picked up to make that call fairly. I tend to toodle over to the dressage whip holder when I am done with my warm up and possibly with my jumping and want to really concentrate on lateral work. Specifically as it relates to my right leg and his desire to lay on it when the request is a bit tough. Now it’s possible that the might have laid on my leg before that, but I’m confident in both his sensitivity to my aids (something I’ve worked very hard to perfect) and my ability to ask very nicely then TELL him with my aids in a manner that he immediately comprehends and complies, that this is a hurdle I expect to be crossed without whip or spur.
So for me, picking it up “when it is needed” is before we start that more difficult lateral work (haunches in, shoulder in, leg yields, few steps of half pass) that I KNOW his ability to execute correctly is going to need just a bit more help, especially if I am going to keep my legs stable.
No doubt I screw up a time or two, but probably not any less than I accidentally do not time a correction w/whip or spur to beneficial effect (we are all human, it happens), so I am not going to sweat that too much. 
I definitely understand the argument for going in to battle with your weapons* and that is the path that clearly works for the most riders and the most horses, but I’m also old enough to allow for different roads to Rome.
- how I hate that analogy - how about not starting a work of art without your paint palette and easel?