How much disk usage do you use? I am wondering would that be okay if I use like 90% of the disk?....

This solely depends on what part of the disk you are talking about, Linux? Which partition are you referring to? You really can't look at the disk as a whole. Could you use 90% home your home partition safely? If you have 100 gigs for your home partition and are leaving 10 gigs as a cushion sure why not.