how to install into staging area correctly?

[Vick Khera]

I have a port locally that does this to install a file so other ports that
depend on this can have something to depend on from the file system.

do-install:
        ${CP} /dev/null ${LOCALBASE}/etc/rwde-base-installed


Lately it seems that the ports does a staging step, and the above file is
not found when building the package for that process. I changed it to


do-install:
        ${CP} /dev/null ${STAGEDIR}${LOCALBASE}/etc/rwde-base-installed


Is this safe to use ${STAGEDIR} unconditionally like this? Should I be
using a different variable to install my file in the do-install step?