We're currently about 1/2 way through the yellow book (but taking a break from Miquon to do a bit more geometry and probability).
When we started, I ordered all the books (I knew it was a system that would work for us and I got a price discount on the full set), the rods, and the lab annotations. Somewhere during our last move, the lab annotations got misplaced and I have yet to find it. We were in the blue book at that point.
I guess that is my way of saying that if $$ is tight, you can definitely hold of on getting it. It is a nice security blanket when you first get started--especially if you don't feel confident in your math skills. It has a lot of activities for getting kids really involved in the topic, but it isn't a "teacher's manual" in the sense that it really explains what is going on in each and every sheet.
Miquon has just about always 'made sense' to me--I could see where they were going with the concepts and understood the sheets enough to explain them to ds.
If you do hold off on ordering it, make sure you subscribe to the Miquon-KeyTo Yahoo group--they are WONDERFUL at answering any questions you might have about specific pages or approaches to topics.
Just my .02