I start in with common IOs at level 12 (lvl 15 IOs), adding in SOME set IOs at level 20 or so -- multi-aspect set IOs (ACC/DAM) at this level are more powerful than SOs, although single aspect IOs are not. But I rarely worry about set bonuses until I get above level 30. The lower level IOs just don't have set bonuses that are attractive enough for me to worry about them.
Above level 30 I buy some IOs for the set bonuses, planning for the future. Since IO sets with good bonuses usually have a few very rare or expensive pieces, I will usually fill in those spaces with cheaper IOs from other sets (aka "frankenslotting") until I can get my hands on the rare IO. So even though I am working on set IOs from level 30 on, many of my powers won't get their full set bonuses until the 40s. Rarely do I wait until 50 though, since my level 50s are generally retired.
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My typical progression is
Nothing until 12
12-21 Lowbie IOs, mix of commons and fcheap sets slotted for value rather than bonuses (Frankenslotting)
At 22 full set of level 25 IOs mix of commons and frankenslotting of cheap sets
If I have a particular build in mind, then at 27-30 or so I start dropping in the 'final planned' sets (level 30-33 IOs)
If I have no particular build in mind, I stick with commons and frankenslotting through to 50.
Theres more than a few of my recent 50s who got all they way there with the level 25 commons still in place. They are plenty good enough for most situations.
@Catwhoorg "Rule of Three - Finale" Arc# 1984
@Mr Falkland Islands"A Nation Goes Rogue" Arc# 2369 "Toasters and Pop Tarts" Arc#116617
Ok, this sheds a little more light on the subject then. I have a particular toon that is 27 now. I guess I should start. But I want to use the reward rolls to get these IO for teh frankenslotting. I get 27s as of now.
Right now, the market is in shambles, but hopefully once I16 hits, balance will be restored. I like to "play the market" when it comes to recipes that I need. With reward merits from TFs, story arcs, etc., you can get some pretty decent recipes that can be crafted and then sold for a pretty penny.
For my toons, I put regular IO enhancements in at level 30 (when it really matters), while slowly adding in IO sets as they come along. If you're not picky/concerned about percentages, you can usually do a pretty good job IOing your toon out if you use enhancements in a 10 level scale. For instance, when you get to 50, work with slotting IO sets in the 40-50 range.
Oftentimes, for instance, a level 45 IO set enhancement is cheaper than its level 50 counterpart. The only difference is maybe a percentage point. Also, use yellow sets. Their bonuses may not be as large as those of the Orange sets, but they're usually a heckuva lot cheaper.
Just my own thoughts 
Well, now that I can afford it and have a stocked Base, I use Common IOs on the way up. Generally level 15s, then replacing with level 25s.
Once I hit 30, I start laying down bids for level 35 Set IO recipes. I accummulate and slot what I can on new slots. Between 35 and 38 I might use a Free Respec to cash out the Common IOs and put in the rest of the Set IOs. While all the early powers are slotted with level 35 (+/-) Set IOs, new powers are slotted with level appropriate Set IOs.
I'd rather not use a high level Set IO in a much lower level power unless it is not going to be paired with another same Set piece so I wouldn't be getting a Set Bonus out of it anyway. The only time I'd use all high level Set IOs in a power is if I'm short on slots, then maximum attribute boosts takes precedence.
Teams are the number one killer of soloists.
I'm trying to get heavy into trickin my toons out with set bonuses. I had this question in mind. Do you guys buy/earn the recipes at a low lvl or do you trick your toon out when it's about to hit 50? I'm wondering if me piling up on these recipes at lvl 27 would be beneficial enough for me to get what I can now as I rank this toon up the normal way...doing arcs and stuff. No farming. Do you get what I am saying?