I don't have Ascension's guide in front of me, but I'd recommend placing the mountains first and adding the rivers later. The reason is that the clouds patterns Ascension uses for mountains are random and you don't know what mt shapes you will end up with. So if you create the rivers beforehand it limits what mt shapes will work with your map. Better to place the mountains first and then paint the rivers to fit them.

Cheers,
-Arsheesh