Suggestions for best use of Leprechaun amulet?

I finally saved up enough buy a Lep amulet.

Any suggestions for getting the most out of it?

Wear it through an entire run? Or, wear my normal one until I get to a boss, then switch?

Beat the boss down to ALMOST dead and THEN switch amulets?

Also, any suggestions for best drop maps?

If its elite, swith when boss is low
If its a Km3 run, having a lep on the whole run is fine