For me personally, I tend to go a little overboard, but then again I don’t think you can ever have too many backups.
Inside my computer I have 6 hard drives. 1 of which is used for backup. Every night all of my important things get backed up to that hard drive. At the same time I also have a carbonite subscription which I have about 20GB worth of information that gets backed up there.
Then I also bought 2 regular 3.5 3TB hard drives, to backup every drive on my system. I use it in conjunction with a Sata dock. 1 of the 3TB drives stays in my office, while the other drive gets put in a safety deposit box at the bank. Both drives get swapped monthly and both drives are never at my house at the same time.
The Sata dock via esata gives you native Sata speeds, thus making for a much faster backup.
The software I use is syncback SE, it’s configured never to delete from the backup and to update files which are new or that have changed. Thus the very first backup takes the longest, after that, not so much.
As far as my quickbooks company files I have versioning setup which saves 7 versions on a usb drive plugged into my Quickbooks laptop (Accessed from all computers via RDP) and 2 computers on my network. Which in turn gets grabbed by carbonite and my offsite backup.