A bidding system that has three possible bids, High Bid, Medium Bid and Low Bid which rotate each month throughout the crew is fair but difficult.
A bid system that only uses seniority as the basic start point, (QANTAS did, I believe), is very unfair in both quality of trip and earnings to junior people until they rise up the seniority pole. Friend of mine with a B747 command in QF, after nineteen years service, could still only bid standby lines, may all have changed now.