I've recently swapped frame, the new frame has a slight shorter toptube. On my previous frame I was running an 80mm stem with my low-risers rolled back slightly. On the new frame I've rolled the risers back to vertical to give a similar reach.

I fancy a new stem to finish the build an wondered whether there was any advantage/disadvantage to an 80mm stem with the bars vertical or a 90mm stem with the bars rolled back. I realise that a shorter stem sharpens up the steering but 10mm is a tiny amount.

Rolling the bars forwards and backwards? That can't be good can it? Most risers tend to have a degree of sweep to them surely by rotating them forward and back it's gonna affect your riding position? You ride with your elbows up and out?

Well, I'm not particularly knowledable, I just go by feel, but I've tended to tweak things by rotating risers slightly. I found this at http://www.surlybikes.com/blog/spew/2_-_bars/:-

The ability to make fine adjustments to overall length, up-sweep, and back-sweep is an invaluable tool for fine-tuning and personalizing your ride. On a typical 40mm rise handlebar, there is easily 20mm of usable fore-aft adjustability and several degrees of sweep adjustability available by simply rotating the bar.

Depends what you find comfortable, set your bars that way and then pick a stem length to suit.

I saw a giant anthem with 60mm stem and flat pedals the other day, so don't be swayed by magazines telling you shorter stems improve handling!

I have always used a 90mm stem on my Cotic but swapped to a 70mm for a few weeks to see the difference, i didn't enjoy the feel or handling at all (120mm forks) so went back to a 90mm.

Agree with the bar rolling thing, it can mess with the backsweep if you roll to far forward

Hmm that surly piece is, erm, let's just say it has the potential to encourage poor bar positioning by suggesting that you could consider rotating risers as a method of extending your "cockpit" (disclaimer: I fuxking hate that word). As thisisnotaspoon says, hold the bars so they are comfortable, so they feel "right" with you elbows up and out, then get a stem to fit. I'm inclined to run a 50mm stem with 680 bars, a longer stem (60mm) felt twitchy.