I'm feeling like I should always have it running since it works pretty well on the machine i'm using (xubuntu 9.10) and I'm going to want to use some audio production applications in future.
But, today I wanted to play a movie in VLC.
The sound didn't work and patchage didn't show 'vlc'. A brief search and I discovered that there is a jack plugin for vlc.
apt-get install vlc-plugin-jack
fixes that... Then within VLC, you need to switch to jack output:
tools->preferences->audio output label 'jack audio output'
then, I had to switch to 'show settings' all.
Open the audio->output modules->jack tree
and check the 'automatically connect to writable clients'
This made it work. I expect I'm going to experience some problems when I have more than one 'writable client', but I'll burn that bridge when I get to it.