The real spring takes all the slack out of the system. Checker springs can “hide” clearance in many places. How hard it pulls keepers into the retainers and any number of different “tolerances”. You check with the real spring, you know what you have. There are so many things that you can’t control or really know, why add any variables to them. If you are doing the job, you have the tools, why not use the real spring. I guess if you run hydr, but not a fan and won’t run anything hydr, but even then, do you KNOW your checking spring isn’t changing what you hydr is showing you?