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I think that I might be about to ask a ridiculous question, but I'll go for it anyway (possibly less ridiculous that not asking and getting it wrong).
I'm currently tinkering with my cockpit and trying a couple of different stems. I have a flat bar with 6° back sweep.
Do I set the bars parallel to the ground or centred in the stem (10° rise); doing it this was could angle the back sweep down slightly but due to my ride position would place the plane of the back sweep pretty much 90° to my forearms effectively placing the heels of my palms on top of the bar. Or is it just a personal preference kind of thing, after all folk tend to tweak the rotation of riser bars?
Whatever's comfortable for you.
Go with comfort, too many variables like bar height, drop from saddle to bars, cockpit length etc for a right answer.
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IMO - slightly upswept - better control, slightly downswept better comfort. Personal choice. All mine run slightly down - its a better position for your hands and wrists for comfort
Personal choice, I ride with a couple of guys who have their bars rolled forward which I find horrible but they get on fine with.