The philosophy that we follow at work is to keep the trunk in a state where you can push at any time without drastic harm to the site. This is not to say that the trunk will always be in a perfect state. There will of course be bugs in it. But the point is to never, ever leave it broken drastically.
If you have a feature to add, branch. A design change, branch. There have been so many times where I thought, "oh I can just do this in the trunk it isn't going to take that long", and then 5 hours later when I can't figure out the bug that is breaking things I really wished that I had branched.
When you keep the trunk clean you allow the opportunity to quickly apply and push out bug fixes. You don't have to worry about the broken code you have that you conveniently branched off.
