I read that the EIGRP stub router should have a default route to the core router. Do I need to configure it manually? or is it created after I issue an "eigrp stub" command under "router eigrp..." ?
You could even just use a default static ip route eg.
ip route 0.0.0.0 0.0.0.0
and redistribute into EIGRP process.
But if you use this or you use ip default-network then you still need to use a distribute-list under the eigrp configuration to make sure only the default-route is sent to the stub router.
If you use the summary-address as in your example then because this is an interface command EIGRP will advertise the summary route only which is exactly what you want.