I think you do have to. Looking at the design checklist on page 563 it says: "1. Select Sophont Pattern." So if you don't already have one, it needs to be made (although I'm sure you can skip the bits about caste, gender, etc. and just concentrate on body form). But, even if we assume you are right, and you can ignore this step, how does one figure out what the characteristics of the robot are? Just choose them out of thin air? So I can make a mouse droid with XS arms that are 50D Str, and for no cost (except the Strong skeleton it would require)? I'm trying to understand how you envision this could happen, so I hope you can clarify it for me. I'll admit if I missed something.
Well, the base assumption of my suggestion is that players and referees keep it realistic. But in terms of actual process, I am suggesting that you first envision what the robot looks like and what it can do. Then build it to meet your vision right up to page 569 section A. Then before putting the final stats on from page 569-A back-calculate the size on page 563 to determine roughly how many dice you need as a minimum for C1, C2, and C3. Then add more size if you need to in order to meet the vision, but with a correspondingly larger and stronger result.
So in the case of your mouse droid with the 50D Strength, and assumed 2d C2 and C3, the total of the three stats would be 54. Per page 563 its roughly estimated size would be 54x12 = 648 units, or about 9 times the size of a human being. Your mouse droid is a true monster! But to keep it the physical size you defined already you will probably have to use 1D for all three of C1, C2, and C3, and even that is bigger than a mouse droid should be.