I wouldn't say the way we do sidings is the most common. In the SCA all american kingdoms, possibly everywhere except lochac dances them differently to this. I'm not sure how other groups dance them, but I get the impression it's at least a mixture, if not comming down more to the american way.
The american way isn't exactly a do-se-do, as I understand it (no passing behind the person). It is a double left forwards, meeting at each other's right shoulders (as we do, but taking longer to get there) and double back. Since a siding right normally follows, you then get to do the opposite movement to the right. It probably fits to the music slightly more easily (since the music doesn't have a break mid double that marks when to change feet), but that's no guarantee it was danced suchly - there seems to be plenty of places where a dance was squeezed to fit the music. There is some beauty in the standardisiedness of it - during an arming your feet are still doing a simple double- but I'm need some convincing that ECD was so standardised at the time.
Either version is equally mildly awkward when doing a siding left with your partner and a siding right with your contrary in heart's ease, although a double forward probably is easier to catch up with if you forget what comes next and start the step late. There might be some other dances for which a clear difference in ease of dancing exists between siding styles.
There are a few other ways to do sidings that have been proposed, some of which might be closer to a do-se-do, or at least j-shaped. These seem to be massively out of favour with both SCA and ECD reenactors. (mostly because they were early reconstructions based on almost no evidence)
Anyway, I think i have the class notes from the KWDS class on this, but i remember most of the brief discussion on the topic (wished it was longer). There's a french dance book of english country dances around 1700, that is supposed to have a diagram of how sidings are done (there's also some verbal descriptions a bit earlier). I think both major schools of sidings claim it supports their version, so it would be facinating to see.
Mind you, I think some backwater sca bit do all kind of strange dance reconstructions, but they would find themselves as confused on the dance floor as i was at pennsic - everyone who dances away from home seems to do the style i described above, and not have seriously enountered other styles.
Tiff 21:39, 10 Sep 2005 (CDT)