As long as it’s somewhere safe (i.e. backed up remotely in source control or by some other method) it doesn’t really matter. If you lose the keystore somehow then you’ll no longer be able to update your app on the android market and will be forced to remove it and re-upload it as a new app signed with a new key.
If you’re worried about security, remember that both the keystore and individual keys within it can (and should) be password protected.
My keystore is in the root of my android eclipse workspace, which is also the root directory of the git repository I use for version control. Seeing as you are developing on multiple machines and would potentially want to do a release from any one of them, I’d suggest doing the same.