>>3769121It's a good rule of thumb. It increases contrast, which is important.
Even if you're not following this rule very hard, it's a good idea to shift your hues somehow across the value spectrum. Using 1 hue just generally looks like shit.
A cool thing to do is to make all your shadows trend towards 1 color, and for all your lights to converge towards a different color. It makes your color palette look more coherent.
You don't have to follow anything you read. It's all just a general guideline from other artists' experiences. Learn from it, use it a bit, and you'll come to understand when it's basically necessary and when it isn't.
