It is never a bad idea to plan ahead of the storm. Exploratory testing is best utilized in the form of an evacuation team. Through early high-level exploratory testing, skilled tester can quickly become familiarized with the terrain and flush out issues. This often includes those issues that might otherwise be found too late, or not at all.
When external QA testing is begun nearing the end of the product’s production cycle, it is valuable to invest a little time into exploratory testing right away. By becoming familiar with the product, the testers will have gained a context for the overall scope of their testing. With this acquired knowledge of the product and existing issues: any potential time adjustments to testing schedules will be discovered early and testing will flow smoother for the first run of test cases and requirements.
Regardless of when testing begins, with the use of excellent QA practices by skilled testers it certain that all areas of testing will be thoroughly covered. But, who would choose to be caught unprepared in a storm?