I mustered the courage to shorten my hoses the other week, last year's (pre-bled) SLX. I found shimano's instruction for shortening without bleeding, and thought I'd give it a bash. The instructions are nice and clear, but I must've got air into the pipe, as there was no power in the brakes at all when I was done. So, in for a penny, in for a pound, and I started bleeding them. (small length of tube off bleed nipple, oil going in the reservior at the top) Just as I was thinking that all the air was out, I kept on seeing a series of small bubbles in the tube. the happened a few times, until I realised that a 1/4 turn of the nipple was making it loose enough to allow air to enter. 1/8 of a turn instead sorted this, but it took a while for the penny to drop, so just a heads up to anyone else trying this. Also, and this seems obvious, but still I managed to fall foul of it, make sure that the reserviour always has some fluid in it!!!! A very easy way to waste another five mins!!!
Hope this helps.