Is Secure Empty Trash still necessary after maverick? With Maverick, hardware is encrypted by default, do we still need to secure empty trash?

Can files still be retrieved after deletion since the entire hardware has been encrypted?
Mavericks does not enable encryption (FileVault 2) by default -- it may (if several requirements are met) offer to enable it during the initial setup process, but it's not the default. Also, even if the volume is encrypted, insecurely deleted files can still be recovered by someone who knows an enabled password (usually any user with an account on the computer).

So, I'd consider Secure Empty Trash to still be relevant (in at least some situations) in Mavericks, even when FileVault is enabled.

However... Secure Empty Trash is not really enough, and hasn't been for a long time. In many apps, when you edit a file, it saves changes by deleting (insecurely) the old file and writing the new version. Securely deleting one version still leaves any older (deleted) versions lying around the disk. So you really need to use the Erase Free Space feature in Disk Utility to go back and make sure they're overwritten.

Oh, and neither Secure Empty Trash nor Erase Free Space really work right on SSDs, just on magnetic HDs. So ... it's complicated.