Sorry for being dense but I am trying...
I guess I don't understand the relationship between rotate/prune and how to implement retention.
If I simply want to keep the last five days of backups can I simply run my backup (--backup) jobs with --rotate=5D? Would I need to worry about prune?
We aren't fully understanding what you mean. If you use --rotate=5D every backup older than 5 days will be pruned.
What did you expect rotation would do?