Only if the installer actually supports installing from extras as well, otherwise an upgrade may be impossible, or leave the system in a broken state.
At which point I really fail to see the point of extras: either it's part of the distro, or it isn't. Labeling it as extras just because it's in a separate CD is useful, because then people can choose which CDs to download and burn upfront, but it serves no other purpose.
-- Stuart Children