From: Mosquito Bite, Texas
Five courts is the magic number you should strive for in as many of your provinces as you can, if you're an expansionist country. You get to have 30 non-protectorate provinces with no risk of waste. Once you get 31, you'll see waste unless you have at least one province at 5 court level. Building to level 5 courts in your provinces is easier than 7 culture.
The advantage of having 7 culture is that THAT province will not experience any waste, no matter how much waste you're seeing nationally. So, if Russia is at 40% waste, but can get Moscow up to 7 culture, then the waste will come from the other provinces. So, you still get the waste, you just get it elsewhere.
Now, here's the coup de gras. High levels of culture do not act to reduce the national waste level, only courts do that. So, while you're preventing those "cultured" provinces from contributing to waste with their own goods, the provicne is actually still adding to the national waste level of 2% per province if its court level is below 5.
That's why building courts is generally a better strategy. But, if you just want to keep your high producing provinces safe, because you are expanding faster than you can build up to 5 courts in several provinces, then stack up the culture in those really good provinces that give you a lot of stuff.
Honestly, I never do that. I always just build courts like crazy until they are at 5. Then I shift the emphasis to culture. I always play very long games. Shorter games require more creative development schemes, depending on the country, and if you achieve Empire status, etc.