Backup of idle machines?

I'm wondering if anyone has developed a strategy for skipping backups of VM's that haven't run since the last backup.

We have a number of machines for test and development that we need backed up, but since some of them are only turned on occasionally so we spend a lot of time backing up machines where the backup is identical to the previous backup.

We could, of course, manually tweak the backup jobs, but I was hoping for something that would say "VM 160 hasn't been started since the last backup, skip it and go on to the next machine".

Any thoughts?

Thanks!

Jeff