If I delete Sparrow set up to use gmail via imap, does Sparrow have any local mail files? Or is everything on the server?
I.e., if I uninstall sparrow, can I delete all Sparrow directories safely?
In general, you probably want to use something like CleanApp for this, but for Sparrow you can actually get this form the App itself.
In the Help menu on Sparrow, you'll see a couple of items "Show Log Folder" and "Show Data Folder". The latter is the local cache of your emails, the former may or may not actually do anything (e.g. if Sparrow's not generated any logs).
There are a few other small files around the place (like preferences), but that Data folder is probably the only one big enough to worry about (e.g. mine is ~1.3 GB). The preferences/etc will likely be a few kB; again, something like CleanApp is what you should be using if you're that fanatical about removing files (;
Bear in mind that if you're using Sparrow for POP3 mail, and deleting emails off the server, this might be your only copy of said emails (i.e. if you don't have another client or another account hosting them).