Understand the anomalies you need to prevent
Read committed, repeatable read, and serializable isolation generally offer different guarantees, and exact semantics vary by engine. Identify whether the application must avoid dirty reads, non-repeatable reads, or inconsistent results across multiple queries.
A transaction that reads a balance and then writes a decision based on it may need stronger protection than an independent analytics query. Consider locking, optimistic concurrency, or a version check when the invariant is specific to a row or operation.
Test contention and recovery behavior
Run representative concurrent transactions and observe locks, deadlocks, and serialization failures. Code using stronger isolation may need to retry the entire transaction when the database reports a conflict.
Avoid raising isolation globally without workload tests. Document the chosen guarantee at the transaction boundary and verify that retries are safe and side effects do not occur before commit.