You can point any entry you are authoritative for to other entries for which you are not authoritative. You can create a CNAME from www.mydomain.com
if you wanted to but you'd never be able to change where www.google.com
itself points. You can create a PTR (reverse) to point your IP to www.google.com
but that won't change the IP of www.google.com
on forward lookups.
Reverse records are in general not "required" but are there to help others determine what an IP is pointing to so are a good idea for troubleshooting by others who might need to contact you. Some mail servers/services refuse to accept mail from domains that don't have reverse records and some go as far as refusing if they deem it a "generic" reverse rather than one that tells it a specific forward record that can be resolved. If you create no reverse yourself you'll usually see a "generic" from the internet provider that simply tells people they own a range of IPs.