Writing tests (or deleting them, refactoring them, etc.) should always involve a cost/benefit calculation, even if it's a rough mental estimate that's not written down. In particular, that requires answering "How much effort will this test take me to write/debug?", "How much extra confidence will having this test give me?", "What level of confidence do I feel comfortable with?" and "Could I achieve higher confidence spending this effort on something else?".