There are two types of passphrase. An encryption for the seed or an extension for the seed. The former is recommended for Trezor as there is an unfixable vulnerability which allows for an extraction of the seed given physical access to the device. An encryption on top of it would make the attack practically useless.

The latter is entirely up to you. The seed extension is not covered by the checksum and you might have difficulty when restoring the seeds. Shouldn't be a problem if you were to make a backup safely.