I'm wondering whether there is any speed (in the OS) or security issues with running postgresql all the time (using launchctl) for development of rails apps, whether I should launch postgres only when I need it or whether I should use sqlite3 for development and postgres for production?
at the sort of usage levels that the average server has - no it won't make much difference in terms of performance. If you have postgresql talking to a web server/scripting language platform on the same box, you can simply not open the port to the outside world
Having your DB running all the time is fairly standard in the server world - so its unlikely for shutting down a DB server to have any advantage in terms of performance unless your system is underpowered for the task (a pII or other museum piece maybe? My dinky little atom server dosen't suffer from running postgres AND mysql at the same time.).
I'm not a dev, but having worked with them, switching platforms can be annoying, so developing with SQlite3 and switching to postgres would probably not be a good idea - at the very least, you can develop AND troubleshoot the postgres implimentation instead of troubleshooting it in production should things go wrong
In short? there's totally no point.