Well, I'm not an encryption "expert" by any stretch of the imagination. In fact, I don't use Bitlocker at all myself. So others are undoubtedly better advisors than I can possibly be on the subject.
I would say simply that it depends, first and foremost, on your own objectives and, for that matter, whether you really need it or not on the drive in question. Generally speaking, encryption's primary purpose is to prevent anyone else from being able to read the data on the drive, and I'd normally assume that to apply equally to a backup (whether in place, lost, stolen, or just discarded) as it would to the original. So, I guess my own inclination, if I did use encryption with that objective in mind, would be to apply it in all such cases. The counterbalancing issue, of course, is convenient replacement if and when that may be required and, for that, the disable/re-enable route (not to be confused with actual decryption) appears to have some advantages. With regard to cloning in particular, see this spiceworks thread
for a lengthy discussion of whys and wherefores, pros and cons, etc.
Regards, Richard V. ("Arvy")