There aren't any site-specific guidelines on this; you can open a casting call pretty much as soon as you feel is reasonable. In the most common workflow people actually cast and wait to receive the recorded voices before they start animating because it lets them tailor the action to the recordings.
As far as politeness goes, the most important thing is just making sure you really plan on making the film before you start casting, and keeping the actors in the loop on what you need from them (ex. if you're going to ask them for more lines a few months after they first record, make sure they agree to that ahead of time). Having a full script together when you post the casting is best so they'll be able to record as soon as you approve their audition. Everything else is pretty much up to your personal preference.