Do it while its dormant, December/January

I have found this link for you, hope this helps you. http://www.oregonlive.com/hg/index.ssf/2009/01/how_to_prune_a_japanese_maple.html

As Bamboo says, leave it until winter. My understanding has been that Acers should not normally need pruning unless the centre is becoming overcrowded, in which case just prune enough to enable the air to circulate a little better.

Or you can do it if you need to restrict the size, or reshape