Can more lands be tapped for mana than needed?
You can always tap any lands at any time that you have priority (any time you could do pretty much anything in the game); it doesn't matter if you have anything to spend that mana on. Note that you can either tap lands for mana as part of the process of casting a spell (or activating an ability), or you can do it as a separate action. But in either case, there is nothing to stop you from tapping for more mana than you need / intend to use.