I only use 'relatime' on data disks. I would not use it on a root
filesystem. /boot might be okay. (I let the installer set the boot, swap,
and root entries in fstab and I am loath to touch them.)
Just in case you're not aware of it, 'relatime' is the default since
Linux 2.6.30, see mount(8).
Just as a data point, I have been running all my disks with 'noatime',
as far as I recall since before the default was changed and didn't
notice any strange behaviour.
It might have been a motivation to stop using mboxes with mutt though
(I'm using only Maildirs for any local storage), which is the often
quoted example of applications that might break with noatime.
As far as I know also Debian's popularity-contest relies on access
times, though it probably works fine with 'relatime'.
Kind regards,
Andrei