fall/increase (by) 5%

[Lenka]

Is there a rule about when the preposition "by" should be used (when talking about percents - falling, increasing, rising etc.)?

I've read the sentences below in different articles today:
1. ... profits rose 11 percent to 2.7bn dollars.
2. Revenues increased 11.6 percent from 229bn dollars to 256bn dollars.
3. They said pre-tax profit soared 18.5 percent at their stores.
4. ... output has fallen by up to 20% in one year.

My question is: when do you use "by" to say how much "by how many percent" something grew/fell etc.?
Do you use "by" just with the verb "to fall"?

[Anglika]

No rule that I am aware of.

In your sentences, you could use "by" in #1-#3. In #4 you need "by" as it is part of the phrase "by up to ...."