What I suggest for most folks is to tie it to a stable branch, that way you get overnight fixes but nothing potentially backward-incompatible. When a major release comes out, you need to svn switch to the new branch.
Many folks in the WP community, including myself, svn up to trunk. I wouldn't recommend that for the general public, but I think it's only broken my site once in the past year and I fixed it by running svn up again. If I was smarter I would script it to svn up, check the site, revert if it was blank. WP.com syncs to WordPress trunk pretty regularly, as well, sometimes daily depending on where WP is in its release cycle or if we have to do any database migrations.
Can you really trust that not to randomly break your site? Surely you'll run into regressions or backwards-incompatibilities at some point?