Ron,
Let me give you an answer with 2 angles:
Angle 1 - On Your Project
Your Cost Management Plan should contain a section that answers this. In that way anyone on your project that has to calculate EV numbers, will know exactly how you are supposed to do it. In most cases, this is often "dictated" by the PMIS / cost reporting software you use, but some software packages are clever enough to allow you to change / set this.
Angle 2 - For PMP Exam Questions
Don't round at all. Whenever you have to calculate a formula you should launch the built-in calculator on your computer and work your way through the formulas using this tool on your screen. Don't do it by hand on paper and don't use a hand-held calculator either. Instead, use the built in calculator, because this is the tool that you will have available during the exam and using it every time as you practice will get you acquainted and you don't have to round because you are using this calculator and you get a "more precise" result. Yes... there will always be the moment when a sample question developer decided to round to 1 digit here, 3 digits there or 2 digits over here and then you will be off. But that is a SAMPLE question and you will not have this problem on the actual exam.
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC