Default "update thank you date" should be ON when printing thank you letters

As filed by community member "clarkac" here: http://forum.civicrm.org/index.php/topic,31220.0.html

When printing a thank you letter the checkbox "Update thank-you dates for these contributions" defaults to 'Off'. That doesn't seem at all logical - if you are printing thank you letters why not update the date? My user would much prefer to have this default to 'On'.