As I understand it, the optimizer calculates the CPU and I/O costs for the possible methods of accessing the requested data, then chooses the access path with the lowest cost. Therefore costing is always “significant” as even small changes in cost could cause an execution plan to change, if the original costs were similar.

The all_rows and first_rows hints are instructions to optimize for fetching all the data or just the first (n) rows respectively. Fetching all the data in a set is likely to have a higher cost, as more work is done compared to returning just n rows. Whether this makes the cost more “noteworthy” is debatable. If my above understanding is correct, the cost is always calculated relative to the other access methods for a given environment. Therefore you cannot directly compare the costs for first_rows vs all_rows for a given statement, as these changes to the environment affect how the CBO makes it’s decision and therefore which plan it chooses.