Should I sign outgoing messages when contacts are not using OpenPGP?

That's a great question and there's some debate as to wether signing messages to contacts who are not using OpenPGP is a good idea or not.

We tend to believe, signing messages in general is a good idea. Often times your contact may not be using OpenPGP (yet). But signing your message allows to verify if the message has been tampered with and check if it is indeed coming from you or an imposter. While both aspects may not be immediately obvious to a person receiving a signed message but unfamiliar with OpenPGP, both aspects can be checked at any point in time once the recipient decides they want to take a closer look at the message in question.

To summarize: a signature adds value and trust to the messages you send and does not do any harm. It may be a good idea, to add a short disclaimer to your signed messages. This messages is OpenPGP signed. To verify the signature and integrity of this message, please install OpenPGP on your system.

See also Why all this hassle?

Signed message as viewer by recipient without OpenPGP on macOS 10.12 gpgm_signed_msg_no_openpgp.jpg

Signed message as viewer by recipient with GPGMail installed on macOS 10.12 gpgm_signed_msg_openpgp.jpg