I did a quick search, and found the American Fancy Rat and Mouse Association (http://www.afrma.org/rmindex.htm).
This page has other pet rat organizations that you can contact: http://www.afrma.org/contacts.htm
Their website discusses pine shaving & bedding, but doesn’t mention Christmas trees in the room.
It looks to me like the danger is from direct contact between the rats & the pine shavings, & the fact that they may eat them, rather than a pine tree nearby.
You should go to the AFRMA website, or one of the other Pet Rat groups, & ask them what they think – but I think that if the rats are on the other side of the room & don’t eat any of the Christmas tree, they should be fine.