In reply to sylvainb:
Adding to what Dave stated, you want to ensure that your regressions are optimized so that you are achieving your coverage goals with the minimum number of tests. Most simulation tools will provide you with the ability to rank your tests to determine their contribution towards your coverage goals. You want to re-rank your tests after each regression so that your are being as efficient as possible.
In addition to using the most efficient tests, you will want to run a percentage of additional tests with new seeds. These will possibly find coverage gaps, or provide a more efficient seed to achieve the same coverage.
I’m not sure what you mean by ‘bug hunting effort spread over time’. I would want to find all bugs as soon as possible so that they can be eliminated instead of finding one close to tape out.