Should a [Ceramic] mug be left covered or uncovered during the tea bag steeping process?
This seems like a case where a very simple A/B comparison will give you the answer for your own personal preference. Just use two identical mugs and do everything else the same except cover one and not the other. If you really want to make it official, put a mark on the bottom of one mug and then have someone else randomize them while you look away. (p.s. not sure why this question is getting negative votes, it seems fine to me).