Do we have to consider the cryptographic properties of the decryption, if we're only using the encryption?
Diffusion is a requirement for a secure cipher, but it is not by itself sufficient. So even if a certain number of rounds has "full diffusion", it does not mean that the number of rounds is enough. In the case of Treyfer, the decryption direction requires five more rounds for full diffusion, but the suggested number of rounds is much higher at 32. (Even that is not really secure, because the key schedule is broken.)

However, if considering block ciphers more generally the question is whether a block cipher can be secure in the encryption direction without being secure in the decryption direction. The answer is probably yes. For example, Luby and Rackoff showed that a three round Feistel network with a secure PRF is secure against adversaries without access to a decryption oracle, but needed four rounds to prove security against adversaries with such access (which they called strong PRP).

So there seems to be a security difference between a block cipher used only in the forward direction and a block cipher used in both directions.