Seems like a good way to take your eyes off of the enemy players.
Not imo..
Its not too easy to miss a big red bar moving towards you, but, hey, everyone's different.
If i don't do that then what I do is watch how much of the minion's hp bar i take down from a basic attack on a melee minion, then on a mage minion, and just guestomate when i should attack.