When first filling a liquid-cooled loop, especially custom ones, it's advised to have it run for 24 hours on a separate power supply, with lots of paper towels in the case to detect any leaks. You'll probably want to avoid using the computer during this time, as leaks don't mix very well with live electronics. If you're using an AIO cooler you wont need to do this test for the full duration, but I would advice running it for a few minutes under close supervision to check for leaks, albeit they are very unlikely.