When asked about my point of view about a guideline concerning code coverage, my answer always is: go for 100% code coverage. 100%, unless… Here is why.
When the guideline for code coverage is 80% (not an uncommon guideline) the devil is in the details. Or in the 20% which is not covered to be exact. When a developer ‘only’ needs to cover 80% of his code, you can expect him (or her) to start with the easiest scenario’s, and work his way towards the more difficult scenario’s until the coverage guideline has been met.