Rule number one. Run the system without the PC one full night. Not just to bleed out the air, but to check for leaks. It's time consuming, but well worth it considering the risk.
What I do is assymble the PC as I normally would but without the thermal compound. Then with all the blocks and pieces in place, I cut the tubes to fit as I've planned over and over again and install it all up. Next, I pull the blocks out with the tubes still on and lay them next to the PC on a piece of light colored paper. I leave the rad in as it's not going to drip on anything the places I have mounted them. If it were above anything important, yank it out too. Unhook the power from the mobo and set the PS jumper. Power on the pump and let it work all night.
In the morning, you can look at the paper and see if any drops hit it. Colored craft paper leaves obvious blotches if water gets on it...at least this crappy stuff I have does. I've found one cracked TDX top and one bad nipple this way, and I'm glad it wasn't in at the time. One drip can run all the way down the board, and the odds are pretty high it'll hit something important and let the smoke out.
Oh, and I tried leaving the cap off a Tank-o-matic and that wasn't smart. I figured I burped cars like this, so what the heck! It splishy splashy'd and made a mess.