Nothing is do dirty that it can’t be cleaned (or attempted). Yes, you should wipe the ceiling fan blades off. Put a sheet or something on the floor. For that matter, you should wipe or clean the motor housing, which is also likely to be very dusty, because the dust in the motor can be harmful to the fan. The primary reason to clean the blades is that when they’re going around they’re slinging dust and all sorts of crap all over the room. Mites, germs, whatever has collected there. If there gets to be too much dust, eventually it may get thrown from the blades in a big clump (nasty!) or cause the fan to be unbalanced which is, itself, a problem. Curiosity: when was the last time these blades were cleaned? Ever?