RJ Zaworski has some principles around high-value testing:
Whatever the tools and approach, software testing proves that software does what it claims to do. Tests help developers eliminate defects, build confidence, practice good design, and ideally all three. They also take time to write, run, and update–time that’s no longer available for other development tasks.
High-value testing seeks to maximize the return on that investment. Like much of software development, it’s as much art as science. But a few practical principles can help keep things pointed in the right direction.
There’s a lot of good advice in this post.