There is no "standard" answer to this question.  You could put reporting
services on either box, or on its own box.  If both of these database
servers have a heavy load, you probably want to put reporting services on
its own box.  Remember that reporting services requires IIS, which will add
to the overhead on whatever machine you locate it on, and may have
infrastructure implications beyond just the database load.

If one of the servers is SQL Standard, and the other SQL Enterprise, and you
want to use forms-based authentication to protect your reporting solution,
but you don't want to buy another Enterprise license, then you'll need to
put it on the Enterprise box.

Again, there are too many variables to put out a standard answer.  Every
deployment is unique, particularly the large-scale data warehouse
deployments.