Ultimately, the system that you sysprepped should be kept until you know you got it right in order to allow you to make changes and re-sysprep if needed. A VM is very useful for this purpose as it is easy to create a snapshot just prior to sysprep and also to backup and keep a copy of the VM.

For a physical system, consider creating a disk image backup prior to sysprep so that you can restore that image in the future in the event that you need to restore and make changes. Note that if you don't want to install disk imaging software on the system you can boot from the disk imaging boot disk and launch an imaging operation from there without ever installing it on the system.