I think it's a given that low quality is better than high quality in both SP and MP games. I also(based on intuition rather than experience) think defeating armies is more important than winning sieges in MP.
The big question to me is which works better in SP, offensive or defensive? I normally set my sliders to high quality to make warmongering harder. Given that I win more battles, but don't have the manpower to assault forts, defensive seems the better choice. However, if I had quality=1 and huge armies, I would probably use assaults much more frequently. If I were using this strategy and going for WC, the siege bonus/malus on my own forts would be immaterial. But would the advantage in starving people out outweigh the advantage in storming forts? I think this probably depends on the size of the forts- small forts you'd want to assault, while large forts you'd want to sit there with the cheaper cannons. For a more peaceable nation, the advantage in your forts would make defensive better.