Pretty old post, so my old answer doesn't apply anymore. Thing is, keyboards nowadays use a cloud service to sync saved words. So when you buy a new phone, or use the same keyboard app on your tablet, your typing profile (which words you type the most, words you saved) are carried over and sync. Google Keyboard, Swype, Swiftkey and a few others of the most popular ones do this.
The best thing to do is to either stick to the default keyboard, or use one that's well known enough to be deemed as secure. Swype for example is I think as old as, or older than Android (I had it in 2008-ish on a Nokia 5800). Swiftkey is very well known, and passed Apple's security review for release on Apple's AppStore. Say what we will about the walled garden, they do patrol their walls effectively. If you use Chrome, Google is already saving your search queries and your interests, as well as your passwords, so if you choose Google Keyboard, it won't make a difference.
