DIS is pretty open-minded on this. What they want to avoid are trolls trying to divert legitimate traffic here to a different site by acting as if they are linking and just trying to help - when they are in fact trying to steal members.
Further, when I link to MiceAge I always state the article is a MiceAge article so readers here know it is a different site and do not mistake the content as DIS content.
I think if you overdo it with links to other sites all the time, someone on DIS might take notice. If you do it on occasion that is more accepted.
HTH