As the others have said, it's a very tough question. I voted that I'd rather know, so that I had some time to 'prepare' myself. I know you can't really prepare and knowing it's going to happen doesn't lessen the pain of the actual event and the loss in any way at all but I think knowing that it's coming allows you the chance to maybe do the little things you've been meaning to do like take extra pics, maybe cut a lock of hair, go for trips to favourite places and give favourite foods (if thats practical) and it also gives the opportunity to decide what you want to happen 'afterwards' (burial, individual cremation etc).
The ridiculous thing is, even with my totally healthy dogs (particularly my heart dog, Sprout) I know it's coming at some point and even the merest fleeting thought makes my eyes fill up and no amount of preparation as above will make the pain any less terrible when the day does come.
I'm very sorry for your recent loss