This really made me nervous but notice the mdstat says "check".  This is because in Ubuntu there is a scheduled mdadm cronscript that runs everyday on Sunday at 00:57 that checks your entire array.  This is a good way because it prevents gradual but unnoticed data corruption which I never thought of.

As long as the check completes properly you have peace of mind knowing that your data integretiy is assured and that your hard drives are functioning properly (I'd say it's smarter than relying on SMART).

[462600.807836] md: data-check of RAID array md3
[462600.807842] md: minimum _guaranteed_  speed: 1000 KB/sec/disk.
[462600.807847] md: using maximum available idle IO bandwidth (but not more than 200000 KB/sec) for data-check.
[462600.807856] md: using 128k window, over a total of 943730240 blocks.


Personalities : [raid1] [linear] [multipath] [raid0] [raid6] [raid5] [raid4] [raid10]
md3 : active raid1 sda1[1] sdb1[0]
      943730240 blocks [2/2] [UU]
      [========>............]  check = 40.7% (384365440/943730240) finish=113.5min speed=82086K/sec
     
unused devices: <none>



ls /etc/cron.d
anacron  mdadm  php5  sysstat


cat mdadm
#
# cron.d/mdadm -- schedules periodic redundancy checks of MD devices
#
# Copyright © martin f. krafft <madduck@madduck.net>
# distributed under the terms of the Artistic Licence 2.0
#

# By default, run at 00:57 on every Sunday, but do nothing unless the day of
# the month is less than or equal to 7. Thus, only run on the first Sunday of
# each month. crontab(5) sucks, unfortunately, in this regard; therefore this
# hack (see #380425).
57 0 * * 0 root [ -x /usr/share/mdadm/checkarray ] && [ $(date +\%d) -le 7 ] && /usr/share/mdadm/checkarray --cron --all --quiet