Macrium Reflect can also be used to do a System Image. IMO, you should always have a fairly current system image available in case of catastrophic failure. Don't depend on the manufacturer's restore disks or partition since restoring from either of those will return you to the way your computer was configured the day it left the factory.
With a System Image, you can restore your system in a short amount of time and have a working system, complete with all (or most depending on how recent the image is) of the software you use. You could do an OS install and then proceed to re-install all of your software but that can be time consuming.
Once you have a System Image, then you need to think about backing up your data. As far as which data to back up, think about what data (documents, pictures, music files, videos and the like) you have and would not like to lose. Any data that's important to YOU should be backed up. How often you back up depends on how much new data you generate each day, week, month or whatever and realizing that any new data you generate since your last backup will most likely be lost in case of a drive failure.
Another thing to think about regarding backups is how critical your data is. If your data is really critical, then you need to make multiple copies of it. Those multiple copies should be stored in separate locations. As an example, if you have your computer and the backups all stored in your office and there's a fire, you will lose not only your computer with it's data but your backups as well.
