It depends on how much weight you have put on as to wether you will go back to your normal back size. If you put on a stone in chocolate during your pregnancy then you will not go back to your normal back size immediately. But you will definately go down from your 38 week back size.
If you started as a 34" then went up to a 36" and are now a 38" then you will not drop 4" straight away unless you survive on endame beans or something. So, by getting a 36" back, you can take it in on the notches at the back as your back goes down.
Does this make more sense?
eg. I started off as a 30" back, went up to a 32" and then a 34". After baby arrived I went back down to a 32" pretty quickly, but I am still not in a 30" and won't be for a while as I need to loose at least a stones worth of lard still.
Another tip would be buy a couple of bras in bigger cup sizes at 38 weeks, then if you dont use them (keep the tags on etc) you can return them to the store for a smaller size if you never get around to filling them up.
Freya bras do some 'pretty' styles for bigger cup sizes. Other wise, Royce is your only option if you are bigger than a G cup. Most retailers stop at an F cup for nursing though. So you have to hunt around.
If you are minted, try Agent Provacteur (sp) they have a gorgeous one for £60 (as worn by Gwen Stefani) out of my league though!