It would be awesomely handy if there was a single button to transfer all resources out of a bottom stage to higher stages.

For example, you're about to separate the command module from a service pod that has batteries/TAC LS/fuel on it for landing. Usually, the command pod will have partially used batteries or life support that could be refilled before decoupling, and manually transferring all resource types can be tedious and takes focus away from the re-entry.