I haven't found anything but wondered if there may be some obscure Reg issue that prohibits giving customers better loan rates if they have an existing deposit relationship?

No, it's fairly common.

Just make sure that you are applying your criteria consistently and qunatifiably. We got slammed for fair lending a while back by not have the deposit relationship discounts precisely defined. As a result, we've made massive changes to our pricing guidelines.

Thanks. Just had a question about it in my gut.