Well, you definitely want an external system image before upgrading.
While you will have the option to roll back for 30 days after the upgrade, there have been reports of this not working.
Not sure if MS have gotten their hands dirty in that.
If you are using an OEM machine, you may have the option to create a set of Recovery disks which will be a copy of the Recovery partition with which to factory reset and you will need a Win 7 System Repair disk to boot up into the advanced boot options to restore with an image created on an external HDD.
Because Win 10 boots up that quickly, F2 and F12 no longer work, so before the upgrade go into the BIOS and set it permanently to boot from the ODD and USB before the HDD, although I believe you have the option from within Win 10 to access the recovery options - you'll need to have a Google for those as I haven't bothered adding those to my notes.