As ATCO91 said
someone with a rough running engine, would just say "I have a rough running engine" instead of actually declaring a pan.
Other pilots would declare a MAYDAY in this situation.
As a FISO I don't consider the priority set by the pilot. I consider the stated problem and react accordingly.
Also I consider how fast a response is needed. If an aircraft is downwind in the circuit and declares a "problem" I may use the crash alarm to ensure a rapid response by the RFFS. If the pilot calls on the radio and states that he estimates being at the airfield in 10 minutes then I would take a less dramatic approach to alerting the RFFS.