Isotretinoin increases sun sensitivity even at low doses, so you want to be aware of that. If you're not in the sun though, then no, you don't need protection. But even if you're just randomly out and about during the day, going to and coming back from lunch, running errands, etc. it's good to be protected.

Yeah, isotretinoin can definitely amp up your skin's sensitivity to sunlight. That's why slathering on some SPF is a smart move.