Design should be validated in proportion to the usability risk there is.
It's not necessary to validate every design via Usability testing externally with users. Some of it is enough to test with internal users – if you have them. And for some things with low risk, it can be enough to validate with experts – e.g. other designers on the team.
Validate based on risk: low risk -> internal validation, high risk -> external validation.
If you want to discuss this note, send me a message on Twitter.