About cleaning and updating commands

Hello there. I would like to ask the following:

1. Is it safe to use the command "yum clean all"? Is there a possibility that I may jeopardize the integrity of the operating system by accidentally erasing a necessary file?

2. Is it true that "yum update" is safer than "yum upgrade"?

3. When should I add "--refresh" at the end of an updating command?

Thanks for your time.