As for your question, it's entirely up to you whether your map is set at daytime or nighttime. Both can work equally well as long as the night maps are sufficiently lit. I've never seen anyone complain about the time of day in a map, ever.
I have, many complain about cp_egypt having a too bright light setting. The same complaint showed up alot on mvm_intercept. The light setting is important enough on that matter. However, with day maps its far less likely to happen that a map ends up too bright, yet with night maps ending up too dark is far more common.
Only if you know how lighting works in tf2 and can optimize its usage well i would go for night, day is easier to make as most of the map is well lit by default to begin with.
I personaly dont have a problem with night maps myself but that has to do with the way the editor for red faction worked (where if you wanted nice lighting you had to start out with a very dark map, because otherwise it would look like fullbright with just a few extra lights). Note that in skullcove there are barely any phantom lights