If they are felted you might not need to. I woudl anyway just because you can use them longer without washing if they are lanolized, and lanolizing will make them softer if they are on the rough side.
It depends on how you want your wool to function. I wash with a lanolin rich wool wash and never lanolize my covers. Here's a quote about wool care from www.fuzbaby.com
Wool covers breathe. This is good. This means that moisture can escape, too. The area around a wet baby can be humid. If humidity gives way to just wet, though, you might try several solutions. Start by changing the baby's diaper more often, even if that means midnight diaper changes. If the wool covers you are using already are not enough of a barrier, you could try: 1) a thicker cover, 2) removing the lanolin that is acting as a moisture resistant barrier and that may be letting moisture slide right off the wool (do this by soaking the cover in warm white vinegar), or 3) you could go the opposite way and smear lanolin on the wet zone of the cover. I have done this, but it leaves a sticky patch and I prefer not to. A heavier cover like the Fuzbomb is your best bet.