Feel free to back up the file but we might also request you to delete it at some point and you should be able to know whether you still have a copy of it or not. It is all-right to lose your share as long as you tell us that you have lost it. It is actually worse to still have a copy of the share "somewhere" while thinking that you don't, than to lose it by mistake.
If someone in possession of a share gets to learn about a very bad event that happened to many of us and really thinks that we are not capable of revoking the Tailssigning key ourselves anymore, then this person should write to the mailing list explaining why she thinks that the signing key needs to be revoked.