/var or /usr for data?

[Brad Waite]

It would appear that the "proper" allocation of filesystems on FreeBSD is
to put all data in /usr.  I'm used to this and have been doing it for
years.

However, there's a few issues that keep coming up.  A lot of the ports use
/var for data dirs.  MySQL, Qmail, dspam are a few that I've had issues
with.

Is there a canonical place to put data files on a modern FreeBSD server? 
Figuring out the sizes for each partition is an exercise in frustration
when I don't know how big /var or /usr are going to grow.

For now, I've changed the default config files for MySQL and dspam to use
/usr/local for data dirs, but is this the "right" thing to do?

I used to put everything on /, but that created problems when I couldn't
fsck the single large partition and I had to boot from CD to fix things. 
That's an issue when the server's not in the same state.

A Solaris associate of mine is of the opinion that /usr should be able to
be mounted RO for security purposes.  If /var was the default for all
add-ons and data, I could see that, but that wouldn't work the ways things
are now.