This is a really good question. There is "normal" (how people normally do it) and then "what really works" (reality of use).
Normally, people put the most important information towards the top of the sidebar, since the belief is that few people will scroll down past the first screen.
What really works is that studies have been done that tells web designers that people's eyes start at the middle, then bounce up to the upper left, scan across the top towards the right, then back across to the left, one or two more times across to the right, then back to the center.
What also backs this up is the old artistic "Rule of Thirds" developed by the Greeks (who wanted to study EVERYTHING) which says that a good piece of artwork provides the eye with a "concert" of movement and resting points. They figured out that the "rule of thirds" (imagine a rectangle photograph with a tic tac toe graph of lines across it) works best. If you put the most visual item at one of those 4 crisscrossing points, the eye is much happier to come there for a rest, then it will scan the scene and rest again on the static subject in one of those "corners". I explain this better in this article.
The top of the sidebar would qualify for one of those "rule of thirds" kinda places, in a loose way. Remember, people see only a screen full of information, not your entire page, so you need to design with that in mind as much as the full length.
Another place that also matches the rule of thirds and is VERY common for putting categories and important links is just below the header, which is about the 1/3 mark down a screen. Or knowing that people do look UP at the top, though usually not for very long as this isn't "natural" since it's so far from the middle, they put their stuff up there. But think about how hard you've looked at a page to find a link to more information and the last place you looked was at the very top. This is an example of "normal use" vs "reality use".
Using that information, we designed our new version of our site to take advantage of the rule of thirds and the "concert" patterns of the eye. Sounds like it takes the "creative" out of the process, but the "how to" is part of the creative process.
Understanding that people look at the center first, then their eye jumps, we designed the header to be fairly eye catching which pulls the eye "up". And to the right. We know the eye wants to go to the left first, but our simple white letters against the colorful background drags the eye over to the right, which means it MUST bounce over to the left (for every action there is an equal and opposite reaction...) towards the sidebar which is set to start about the rule of thirds point, and the road in the background leads them there, hopefully with a sigh. ;-) On our front page, we have the most important pages/links people look for listed at the top of the sidebar, but on the rest of our pages, we have a photograph that changes with every load. The road leads to the photograph, which helps to guide the eye, and below it are the important links. From this point, the eye automatically must move to the right, and there is the title and start of the article, just waiting for them.
Before, I always had my categories there at the top of the sidebar, but we decided to change them to the very top of the screen. I'm not happy with that, because I feel that people are now looking harder to find the categories of information, but for now, we are going to try this and see what happens. We might move them to the spot we now have a "pardon our dust" note. Always tinkering.
I recommend that you test this theory yourself. Visit some top drawer websites who have paid professionals who research the snot out of everything to figure out how to lure people through their site. Microsoft, Google, Yahoo, PCMag, etc. come first to mind. Microsoft, with psychologists and analysts helping them design their pages, have mastered the concert of the eye. They have so much information to share, they work hard to lead the visitor without overwhelming them with information.
As you visit these pages, watch where your eye goes. Pay close attention to the motion of your eyes across the screen. What draws your attention, how quickly does the eye move, where does it rest, what holds your attention, what do you skim over.
I know it's a long answer, but I teach nature photography and have spent years and years studying how someone judges, examines, and likes or dislikes photographs, as well as what makes one photograph sell and one not. So it's a fav subject. And one we can all learn from.