Mandatory fields are always a problem when you have an interactive selection screen. I never set fields as mandatory or obligatory when my selection screen is interactive rather I do the checks that I need to do in the AT SELECTION-SCREEN ON events and even then I only do the checks if the user has pressed a button that would result in the report going 'ONLIne' - ie the green tick, process in background or save as a variant.
In that way the annoyance factor of filling mandatory fields disappears and the user can mess around with the entries on the screen to his hearts content without having to enter dummy values (for example).
Rich