I pretty much buy a whole set if I like it, which I generally do since I love gold shop items. Though if I'm trying to save some gold here and there I try to restrict myself to colors and/or items in a set that I think I'll use the most.
I very rarely buy a whole set of anything. And even when I do I never EVER wear a set. I'm more likely to buy every color of one item.
Call me elitist if you want, but I just can't stand it when I see people wearing a set. Whether it's every pose in one item to make the whole outfit, or something they bought at one of the gold shops.
It's just lazy avatar creation IMHO and I just don't see the point.