I do not use Purples. I sell them. I love ATOs, almost every toon has them, I don't buy them so I have to play for them. After that, the below rules apply.
My slotting is defined largely by how the character plays up to level 25 or so. Its easy to get there in fairly short order without a farm or a PL. This is also because the majority of the "good" Set IO's begin there slotability at 30 (I known there are many that are early, especially ATO and PvP), so that's my break point. So I play them freeform, with no real Mids work to level 20 in order to feel their play and determine fun. If they make it there then I frame them up in Mids and see what the numbers look like. If they look mediocre with a Set IO then I know they aren't worth the full investment but if they are fun they get flagged as Partial Set IO. There I give hem Set IOs that will impact positively the mediocre and enhance the fun; recharge, or some procs, things that bump recover up a little.
The ones that look real good in Mids are moved to test and filled to see how it flows. I may have kitted them up with solid defense but if it was at the expense of play flow, I might rethink things. Anyway, if they pass this then I transcribe the buy list and then drive on playing. I am not in a rush to super charge them so for the more expensive ones, I will put the order in that I am willing to pay and wait. Also I use Enhancement Converters to turn what I don't need into what I do need.
It works for me to where they all play well and I am not broke. In fact, I have two (2) Level 50's that are still slotted only with SO's and a handful of IOs (Doctor Finetush and Flattery). My crafting toon is working toward all Standard IO for them but that is also a take my time thing.