there is no specific rules here.
But it is common to us the same ip for both.
This is also what Cisco does :
C:\>ping cisco.com
Pinging cisco.com [198.133.219.25] with 32 bytes of data:
Control-C
^C
C:\>ping www.cisco.com
Pinging www.cisco.com [198.133.219.25] with 32 bytes of data:
Control-C
^C
C:\>
Same ip address for both www.cisco.com and cisco.com
Gilles.