It's definitely fine to do with the proper tools. It isn't exactly rocket-science, after all. However, you will leave lots of metal shavings, so best to work on an empty case or a side panel removed & then thoroughly cleaned afterwards.
I added a blowhole in a case with a metal cutting hole saw, easy-peasy. Used a deburring tool (a round file is fine), then sandpaper. It's also a good idea to put masking tape around the circle to help keep it neat. I started the center hole with an awl & a small drill bit before moving to the hole saw. You can paint or permanent marker the bare metal though usually the fan guard/filter covers it.