De acordo com o seguinte HOWTO, seu .bashrc é realmente o melhor lugar para ajustar $ PS1:
the PS1 string should be set in
.bashrc. this is because non-interactive bashes go out of their way to unset PS1. the bash man page tells how the presence or absence of PS1 is a good way of knowing whether one is in an interactive vs non-interactive (ie script) bash session.[...]
therefore it should be stressed that
PS1=..blah..should be in.bashrcand not.profile.
Fonte: link