The difference between XP and Vista/7 default ownership settings relates to the introduction of UAE (better security). Under UAE an administrator is effectively demoted to a limited user account, thereby, restricting any administrator account's ability to change OS settings for files not owned by it. When UAE detects a change requiring administrative privileges, it prompts the user to escalate the account's security token to the increased privileges offered by the account's role as an administrator. You can either decline or accept the UAE request. Unfortunately, even when running with UAE, a demoted administrative account can still affect OS settings by changing files it owns. Ownership of a resource grants full access to this resource even when other permission settings do not. To circumvent this security hole, Vista/7 files created by any specific administrator are now owned by the Administrators group. Therefore individual administrators can no longer change any files without first being promoted to the administrators group.
Before UAE in Vista/7 you could effectively simulate this scheme by using a program called "Drop My Rights". It was developed by a MS engineer and freely distributed by MS. However, before installing the program, you needed to change the registry settings to establish the default Administrator owner to be the Administrators group, so future program installs would set the Administrators group as the owner, as well as alter the file ownership of files in the Windows and Program File directories using the subinacl.exe utility to change the ownership of existing files to the Administrators group.
I would not change the default owner setting unless you wish to introduce a security vulnerability.