There are some research suggesting content should be put where the user focuses first: top-left. Of course, that's for users in which cultures the reading order is ltr.
I believe them to be somewhat wrong. Yes, a ltr-user would first focus on the top-left point by default. But, after quickly orienting him self, he'd expect some introductory stuff on the left or on the top, and the main part to follow to bottom and right. A sidebar on the right would distract the user, as the content would have to fight for the user's normal tendency to go over all the way to the right.
Like with reading, you don't find your self returning to the left page when reading the right one, since you already know what's there, yet you'll be tempted to peak at the right page when reading the left one.