If you are an enterprise shop, you likely have so many disks spread across so many servers that you probably have an admin team responsible for projecting utilization for the next 12 months, so that procurement and installation costs can budgeted.
For the home user, or a small business, 90% is still a good rule of thumb. I would hate to see some additional process running in the background constantly projecting when the disk will be full. Just throw a warning for the user when you reach 80-90% capacity, and let them figure it out. They are probably more likely to fill their thumb drives than they are the local media.