gre tunnel config...

in the ccie r&s cert guide wendell uses a configuration example whereby the source is a source interface, and the destination is an ip address... i can't imagine that this is some kind of best practice situation... at the end of this chapter he restates the configuration commands where again, the source for the tunnel is an interface, ie, loopback 0 (as if there are no other choices, see below), and the destination is an ip address...  i'm not clear why he is insistent on this point...

perhaps you save some typing, i don't know...  i like to always use an ip address for both source and destination... 

these are the facts...

DLS1(config-if)#tunn source ?
  A.B.C.D            ip address
  Async              Async interface
  Auto-Template      Auto-Template interface
  BVI                Bridge-Group Virtual Interface
  CTunnel            CTunnel interface
  Dialer             Dialer interface
  FastEthernet       FastEthernet IEEE 802.3
  Filter             Filter interface
  Filtergroup        Filter Group interface
  GigabitEthernet    GigabitEthernet IEEE 802.3z
  GroupVI            Group Virtual interface
  Lex                Lex interface
  Loopback           Loopback interface
  Null               Null interface
  Port-channel       Ethernet Channel of interfaces
  Portgroup          Portgroup interface
  Pos-channel        POS Channel of interfaces
  Tunnel             Tunnel interface
  Vif                PGM Multicast Host interface
  Virtual-Template   Virtual Template interface
  Virtual-TokenRing  Virtual TokenRing
  Vlan               Catalyst Vlans
  X:X:X:X::X         IPv6 address
  fcpa               Fiber Channel

so given a configuration task would it be incorrect if the question stated "use the loopback" as the tunnel source, and you used the ip address of the loopback instead...

for me they are equivalent...