The gloves from Ragnaros are the best offset piece to use since they are half a tier higher in ilvl.
Once you move into heroic modes it gets a bit more tricky and will vary depending on what is available to you. Until you are downing heroic Rag, it may be better to use a 391 random helm from Ryolith, depending on the itemization of course. I have all 5 pieces just so I can swap them around as I acquire more 391 gear. Right now I'm using the H Alys legs as my offset piece, since the tier legs are the worst itemized (and I can't get a token off Shannox to save my life). We haven't gotten a helm drop off Ryolith, but when we do, I'll probably swap that in and go back to my tier legs. We are weeks away on downing H Rag at this point, so I'm not too concerned with those pieces. Simcraft your gearset options and that will tell you very quickly what is best for you.
TL;DR / Didn't go to link: The best combo is the Heroic T12 Healing Shoulders + 4p in other slots with Heroic Flickering Cowl (Haste/Spirit) + 4p being a very close second (5PP lower). The worst combo is Heroic Flickering Cowl (Haste/Crit) + 4p being ~43 PP lower than the 'best', with all other combinations falling somewhere in the middle. Let me remind you that our red +40 int gem is only 40 PP, to put it in perspective. As I go over in more detail in the link, if you aren't killing/close to killing Heroic Ragnaros, your best option is going to be Heroic Flickering Cowl with the other 4p.