AFAIK, Shimano doesn't say either way for square tapers and yes for Octalink. Campagnolo says absolutely not for square tapers.

However, a mechanical engineer I know who is an expert on clamped interference fit parts says grease on square taper cranks is definitely a good idea. What he also says is to torque the crank to spec once when the arms are installed and then leave them alone. The damage is done by repetatively retorquing the fixing bolts which "chases" the arms up the taper until they are distorted or crack.