Rats have very very delicate respitory systems and the oils in pine and cedar cause irritation. There is also evidence that being housed on pine causes a rise in liver enzymes in rodents. Here's a non-technical explaination of it: http://www.ratfanclub.org/litters.html. I can find you a higher-level overview if you are interested.
If you are just feeding them off, I suppose it doesn't matter. But if you are going to keep them as pets or care about their long term health I would advise using aspen.