[cc rider]

You probably wont notice a difference with it open or closed. The proper way is with it open cause its more accurate. It has to do with air. The main thing is being consistent. Do all cylinders the same way, not one with throttle open and the other closed.
Only you can decide if its worth repairing

 

[cc rider]

Well I guess it depends on the bike because I just did a compression test on my son's cbr last summer and didn't open it at first. Then I remembered and tried again and didn't notice a difference