It doesn't necessarily follow that the best time to trade is when the currency is most active - that's going to depend entirely on whether your strategy is designed to benefit from the increased activity.
The last time I checked (over a year ago) I was surprised to discover that there was as much short term volatility in the Euro around the time of the US open as there was around the London open (although there was more volume transacted around the latter).
This may no longer be the case.
I would advise that you find a way to carry out your own data mining for this kind of research (can be done in excel or most decent charting packages). For instance, I'm just an anonymous bloke on a forum and everything above could be completely made up! It's really important to test things thoroughly for yourself and see what works before you put money on the line. Never trust anything just because you read it in a forum or a book etc. Be skeptical.
Hope that helps.
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