For starting a new load, I'll agree with bronco -- if the book lists Win primers and I have CCI, I'll not bat an eye and use the CCI primers.

However, if I have an existing load that is on the hot side of the chart that is made up of X primers and the only primers that I can buy are Y brand, then I'll drop down a grain or two and work up a new load with the Y primers. I'll do that both for safety and accuracy reasons.

Changing any component in an existing load can affect both pressure and accuracy.