Doesn't really matter, although it looks better if they do match. I remember getting told off because my girlfriend at the time asked me how she looked in some underwear she was modelling, The first thing I said is, they don't really match though do they? She actually wrestled me. :L
Depends on the situation. When they match it is def. sexier. In a casual situation like you two are just hanging around it doesn't much matter but if you are dressing up anyway then go ahead and throw on something that matches. It kinda shows you were thinking about being seen in them and that a good thing.