I use ubuntu all the time, I don't remember having to configure shmmax to get it working with my blog. I think you only need to mess with that stuff if you're trying to increase the number of connections or "scale" your blog. I think 90% of the people who use mysql, use it without touching a configuration variable, and those people could just as easily use postgresql without configuring kernel.shmmax.
I don't know, but I'm guessing that 90% includes people messing around with django/rails/lift etc, or they're running a blog/cms/wiki on some shared virtual hosting somewhere.
I worked on MySQL before I had to work on PostGres and these were the major turning points in my understanding. Hope it helps someone else.