On one hand: With no attic space, and no heating, the inside and outside temps would remain (ABOUT) equal, so I would say you can skip the venting.

On the other hand: It depends on what you're doing with the shop. If you will have alot of moisture, it may not hurt to let it out, but roof venting isn't the only way to accomplish this either.

Your question was are you REQUIRED to vent it, and you are not.

If you work in the workshop in the summer time, some venting may help the breeze come through better. As already pointed out, you're not required to have venting per IRC §806 unless there's enlclosed attic or rafter spaces. If you're open from the floor to the underside of the roof deck, no venting required.

My thought is....Why wouldn't you want to vent it? Any chance to get rid of any heat build up and/or fumes would be a definate plus.