First, remember that "trunk" is just another branch in Subversion, and it has no significance whatsoever, so admonitions to be on a branch rather than on "trunk" are meaningless. What they're trying to say is, "Don't do your development work in the stable branch."
So, so true ... the most general way i can think of to express the sentiment is: "commit to a branch that is no more stable then the changes you are commiting"
- if the "head" is an extremely stable line of code, only commit to it once you are confident your changes are fairly stable and won't need any more tweaks
- if the "head" is where lots of iterative development is going on, then commit to it early and often