Nope, you should be just fine sharing your Threema-ID. The reason being that your communication is protected with end to end encryption.
Strength of the encryption: The asymmetric ECC based encryption used by Threema has a strength of 255 bits. Accordingto a NIST estimate (page 64), this corresponds at least with the strength provided by 2048 bit RSA. ECDH on Curve25519 is used in conjunction with a hash function and a random nonce to derive a unique 256 bit symmetric key for each message, and the stream cipher XSalsa20 is then used to encrypt the message. A 128 bit message authentication code (MAC) is also added to each message to detect manipulations/forgeries.
Sharing your ID is akin to sharing your public key. Unless someone has the private key, your communication will remain secure, regardless of who has access to your public key. Threema does all encryption and decryption of messages locally, according to its FAQ, so your private key remains in your possession at all times. And if you want to communicate securely, you need the other party to access your public key.
On top of these facts, you don't have to reply/add anyone who you do not trust. So if anything feels fishy, you can just nuke that person from your "friends" list.
More info on the encryption techniques Threema employs can be found here