I prefer max even on WC runs mostly for faster siege ability and having high professionalism saves manpower in the longrun because of better troops.
But this is considering running quantity ideas, and once your empire is really big (like 3000 dev or more), exploit development serves the same purpose and won't ruin your manpower pool too much since you'll just be conquering more anyway. Paying 125 sword mana for manpower is also a decent trade off, and more valuable than two artillery barrages in my opinion, since good players on normal difficulty probably won't need more than quantity ideas for their WC's, and perhaps offensive if they really hate sieges.
I say it's optimal to get 100 professionalism and sack it for manpower once you have 125 sword mana to spare. Over the course of a game, it's really easy to grab from recruting generals and events, and manpower can be gotten in other ways, namely quantity ideas, building barracks in your states, and exploit development once you have so much development that it no longer really matters. On normal difficulty, you should only have to do it 2-3 times if you don't manage your armies well.
On VH it's probably a different ball game, where manpower is more important, but at least with quantity ideas, careful army management is usually enough except the largest of wars. And when you have 2000+ of manpower development, it's easy to grab a quick 50k if you need it from exploit development (which you'll just be replacing anyway with further conquests).
It's also worth noting that max army tradition plus offensive ideas gives 40% siege ability, and if you take diplomatic ideas, having a spy building a network ahead of time is easy to arrange. 40% siege ability shouldn't be discounted on WC attempts, as your sieges will be going almost twice as fast which makes chain warring Ottos, Ming, and late game HRE far more efficient.