I prefer it always, I'm not a fan of "always clicking to check everything" it doesn't make you any better at the game because you can click more things faster.

I would really like it if there are multiple champs in an area and you don't have the time to check mana on all of them real fast before the fight because you are already in the fight. If you know Ashe's ult should be up and she has full mana and someone else has almost empty, you're more likely to gank her leading to a more successful assault that allows you to assess the situation and act accordingly.

I'm the OP and I still stand by Mana bars right underneath the Health Bars, only smaller but with "Steps" like the Health Bars have.

I must agree that visible mana bars would be a fantastic addition. Yes, you can click the enemy hero to see his current mana, but is this tedious? Yes! The lack of a mana bar just makes us do more actions to find the same piece of information. Since this piece of information is already available to us, I see no reason why it should be more difficult to access.

I always felt that this would be a useful feature in DotA that could not be implemented due to Warcraft engine limitations.