Bag the tattoo machine or not?

I have a question regarding the practice of bagging the tattoo machine. It seems to me that is the sanitary thing to do. However, there seems to be much footage or photos of tattoo artists (often enough pretty well known names too) who don't put a bag on the machine...sometimes they'll just cover the power cord that leads to the machine. So just to clarify for myself...is bagging the machine absolutely necessary, and if not, how is a sterlized machine guaranteed? Thanks in advance!