Q.Do we need to keep the steering gear pumps on once the vessel is alongside ?

In my opinion, we do not generally need to keep the steering motor on while alongside. But while alongside at a berth/port with current (such as in river ports), it is strongly advised to keep at least one steering motor on.

This would prevent the rudder to move away from its mid-ship position due to current.

If the steering motor is not kept switched on while along side a berth with the strong current, the current may bring the rudder hard over to one side. This may put extra strain on either the forward mooring or aft mooring depending upon to which side the current has put the rudder.