Hello Chris,
Good question. If you use eBGP and the same AS number on the spoke routers then the route-map isn’t really needed since the spoke routers will deny prefixes with their own AS number. On the other hand, the hub will advertise those prefixes so a route-map isn’t a bad idea…it ensures that we really only advertise the default route.
If you use eBGP and different AS numbers on the spoke routers, the route-map is a good idea.
With iBGP, it’s also not really needed since because of split horizon, the hub won’t advertise a prefix from one spoke to another. The route-map does prevent advertising a prefix in case the hub would learn some prefixes from another BGP router…perhaps some eBGP router in another AS that is connected to the hub.
Rene