Although the size and shape of the crew hatches varied between Sherman production models, the official procedures for mounting and dismounting the medium tank were for the crewmen to use the hatches/hoods for their respective positions. This pdf of an M4 training manual has the drill for mounting and dismounting starting on page 10. Of course regulations and operational procedures can be two very different things. This training video does show that the sighting hoods for the early mark Shermans were quite cramped while some of the later variants had roomier hatches, as seen in this photo. Sherman crewman undoubtedly had their own way of doing things (like many other soldiers past and present) including mounting and dismounting, but the regulations were a logical solution to entering into an armored steel box quickly, especially since it was not easy to access the turret from the two front positions as seen in this picture.