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Search engines handle underscores in different ways, so it's best to make them uniform and use hyphens, all being equal - This appears to be best practice in Google's eyes. I wouldn't worry about it too much thought and I would take vitalscom's advice if you were thinking about moving them over.

but it's still user friendly.

Thanks Chris, I just used that tool. Looks great. And now allows 2500 domain checks.