I always sharpen a second time if and when I convert to jpg, which is usually for uploading to the web. When I convert to jpg I always apply an High Pass sharpen as I find this method does not increase any artifacts.
A good read on sharpening can be found by going to Cambridgeincolour. Well worth reading, but you may have to supply an email address.
http://www.cambridgeincolour.com/tut...sharpening.htm
Harry