For me, properly slotted is based on my MIds final build for that toon. So at the moment, none of my builds are 'properly slotted'. Mostly because I only recently started playing again on Homecoming so I don;t have any level 50 toons yet.
Below level 50, I tend to slot towards that final build, but will also put slots where I think I need them as I go. Running out of end - more endreds, missing a lot - add some accuracy, etc..
From lvl 22 onwards, I start slotting IOs instead of SOs or DOs, focusing on the powers that I feel need them.
I used to leave enhancement sets until 50, but if I understand correctly it is now possible to upgrade slotted enhancements so might have to rethink that.