Rationale
The API version used by your metadata is specified using the latest (or a sufficiently recent) version of the API is generally a best practice to leverage the most recent features and reduce the risk of bugs.
Scope
Metadata files
Best practices
Align your team to a recent enough version of the API to leverage most recent additions to the platform
Be aware that LockerService is mandatory starting from API version 40.0, we suggest to use at least this version as a default.