It would be very useful to be able to sort subforms. I often use a workflow to pre generate a large number of blank related records that so they can be quickly edited in a subform.
For example, there might be a large number of locations in a warehouse that need to be inspected. Each location has a location ID, and I would like to sort the location inspections by this ID in the subforms so an employee can walk past all the locations and enter a value without having to open and save an individual form for each location.