With FileVault on, is secure trash emptying needed?
Secure Empty Trash and FileVault are two different methods for protecting data. FileVault encrypts everything on the hard drive. Only someone with an admin password can decrypt them. This includes anything in your Trash. So by default, the files you delete when FileVault is on are safe via encryption. Even if someone recovered them, they'd still need your password to decrypt.

Secure Empty Trash has nothing to do with encryption. The default Empty Trash just deletes pointers to old files and marks the space they were using on the hard drive as free to use in the future. However, the files are still there if someone ran a data recovery tool or until the OS decides to put a new file over them. Secure Empty Trash prevents recovering deleted files by writing data (zeros) over the space the files you're deleting were using. Meaning the files are completely destroyed.

Here's how it breaks down:

FileVault On | Empty Trash (non-secure) | Someone can still recover those files, but they will recover files that are encrypted per FileVault

FileVault On | Secure Empty Trash | No one can recover the files, so it doesn't matter whether they were encrypted or not to begin with.

Do you need to use Secure Empty Trash with FileVault on? I think it's overkill unless you're afraid that someone will recover files and have your password ready for decryption.