No, you do not need Kimberly Einmo's rulers to make her quilts. That said, I have two of her rulers and love them. I use her Jelly Roll ruler every day. It is 5" x 10", a perfect size for pre cuts. The markings are easy to see. The only problem is that the markings are in 1/4" increments so I could not use it when I had to cut 2 7/8" squares today. Her jelly roll ruler is my favorite ruler and has been well worth the money.
I also have her EZ Flying Geese ruler. You can use the Fons and Porter Flying Geese ruler if you have that. I think Kimberly's is marked so that it is easier to use. Actually, I seldom use a ruler for making flying geese so you could probably get by without a flying geese ruler.
If you are watching your spending, buy none. If you use lots of pre cuts, the jelly roll ruler is well worth the expense.