I'm glad it's helpful!
Defining it and then invoking it in ~/.Rprofile would work, but then
you will need to be careful about managing both ./.Rprofile and
~/.Rprofile files. If you have one of the former, then the latter
does not get sourced at startup (see ?Startup). Of course, you can
put source('~/.Rprofile') in a local ./.Rprofile to take care of
that if you want.
But in the long run, it would be a better practice to put personal
helper functions like this in a package and then load it in your
.Rprofile file(s). Most of my ./.Rprofile files have
require(rmacq)
setwid()
in them (along with whatever other directory-specific startup
actions I want). The more personal helper functions you have, the
more valuable it will be to put them in a package instead of
defining them in ~/.Rprofile.