I don't charm an item unless it has either 3 or 4 slots and I am putting in gold charms. That way I know that I will probably use the item until the end of the game. So basically I have been just starting to put in charms in Act 3.
Up until the time I start putting in charms I collect all the 3 and 4 slot items I can find, then select the best one and start charming it. I also keep the 2nd best of each piece of gear as a backup in case I change my mind.
