IIS 6.0 allows you to create application pools, then each application pool can be configured to run under a separate identity. When my company deploys web sites, our practice is to create an account for each web application and then create a new application pool to run under that account. This approach however doesn't work with Reporting Services, you might end up getting weird errors like:
"The report server cannot decrypt the symmetric key used to access sensitive or encrypted data in a report server database..."
"Key not valid for use in specified state."
This happened to us with our first RS deployment. After many hours of frustration we opened a support call with Microsoft, then it still took some time to finally diagnose the problem. The bottom line is that RS in IIS 6.0 needs to run under the predefined Network Service account, you cannot use a configured account.