It was all about visibility. The only person who would have his head out during combat would be the commander. Sometimes being able to see around you is safer than being buttoned up.

I can cite a story told to me by a Sherman veteran that supports this, along with the fact that commanders, even to this day, have a higher casualty rate than other crew members.

Tom, the veteran I talked to, told me about how his commander was killed by a mortar round impacting on the top of the turret while they were in combat. The commander had his torso outside which was completely shredded.

So unless under artillery fire, it is appropriate to have the tank commander at least sticking his head out the hatch.

EDIT:

I apologize for the lack of written sources. My reasoning was that nobody else seemed interested in answering OP's question and I believe my post to be accurate. Although this is a sub for historians to answer in a very academic manner, I believe sometimes a quick answer sourced from general knowledge is sufficient for such a specific question.