The people that complained about you medding are just wrong.
That said, if you're sitting there at full mana not doing anything, that's wasted mana regen. It's best to cast something when at full mana, such as a nuke or a re-buff. However...I don't think it's that common to be sitting at full mana for very long.
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Hitting stuff while not FM is probably not a good idea, though, past level...14 or so.