I test all the repairs on fresh installs of each version of Windows. After all it is meant to make things work so it shouldnt hurt anything running it when you like.
I myself run it when I need it or when things dont feel right. I always do a registry backup, because there is always unknown variables, so in case something goes wrong I can restore the registry.
I also always run the repairs twice and in safe mode. So run once, reboot and run again. this is because sometimes some repairs commands cant or are unable to run because of something else, such as a permission or something, so when the repairs run the first time it may have fixed it so that the repair commands that failed are able to run now, but the thing about windows is it has to be reboot to make all the changes to hold. then when the repairs are ran a 2nd time those commands work like they should.
Shane