My DD has always napped in a darkened room and still does. My MIL wasn't convinced by it at all and there's been a few times when she is looking after DD that she has only short-napped because she didn't darken the room before putting her down. I know the dark room definitely helps my DD to fall asleep quicker and get through those 45 min sleep cycle transitions.
All that said, I don't think there is a right or wrong way, just what is best for your LO. My friend's DD always napped in a light room. As for napping on the go in the buggy, have you tried a snoozeshade?
HTH: