First of all, if you are not using iTerm2 already, I suggest you download this free software as a replacement for Terminal.app.
Among many other extra features, in iTerm Preferences, you can configure the default working directory for new shells. You'll find these settings in Profiles - General.
The advantage here is that you can set different behaviors for opening a new window vs opening a new tab or even creating a new split pane. I personally like staying in the current working directory when adding a split pane, for example.
You could do it in your startup script as well (though I find the iTerm solution cleaner, or at least more flexible), but then I advise you to use
~/.bashrc is not automatically sourced when opening a new shell window, and
~/.bash_login is only sourced if no
~/.bash_profile is present.