I would like to know if it's unsecure to leave the .svn folders accessible to anyone on a webserver
if not what's the best way to secure these?
SVN export is a better answer for most people. Metadata leaks could be problematic in some cases, but if anyone was really surprised by them they didn't read the fine manual and should be scolded. For the rest, I'm using this in .htaccess and it seems to work okay:
## Need to tell apache2 to ignore .svn ... ?
RedirectMatch 403 /\\.svn(/|$)
This has shown to be a security hole, and hackers have been able to view the source of some sites this way.
Typically for sites in a production environment you should use a svn export instead of a checkout.