I've seen the drive gears lubricated with grease before but I personally would not grease the outer drive gears,I know the wheels have dust seals but dirt and debris eventually works it's way in and will stick to the grease if the gears are lubricated.
YouTube Video showing lubrication of the inner drive gear and I would grease the wheel bearings and / or bushes.
A couple of pics below mentioning the drive gearbox assembly grease specification and lithium spray can be used on axles or grease.
Not sure of other other people's views about using grease on the drive gears but I think if you use grease the debris sticks to the grease and if no grease is used the debris can find it's own way out past the dust seal.
