"When do O-rings need maintenance?"
O-rings need regular maintenance. Having said that, you should also not overdo it. O-rings need to be maintained flexible with SEACAM silicone lubricant. Grease doesn´t not have sealing properties, in too large amounts it only does harm, attracts dirt and sand and can therefore result in a defect in liquid tightness. You should only grease those O-rings which you can reach without dismantling the housing or the port.