A word of caution. - Because deionised water has had all the ions stripped from it, it's unexpectedly corrosive so not a great thing to add to a radiator.
Think of it this way: Tap water usually has a load of ions in it (calcium, magnesium, chloride. fluoride, etc), and although it's not saturated with them, it's a pretty stable solution. Take them all out, and there's plenty of room to pick-up more ions (such as aluminium and iron from a radiator or engine block). - Mixing a corrosion inhibitor/anti-freeze serves the additional purpose of preventing DI water from attacking the metal.
When I did my coolant change, I used Fricofin coolant and distilled water. Tap water contains all sorts of stuff to make it drinkable, plain old H2O tastes pretty awful. The salts and minerals in tap water can and will react with different metals and alloys, the result being rust and the water turns into electrolyte which you don't really want in a cooling system. One of the reasons you need a good engine block to chassis earth is that the water/electrolyte in the system will in time start to eat the inside of a cast iron block if not properly earthed. I have seen cylinder head studs eaten away inside the block to the point of shearing off when torqued up. I refer more specifically to straight six Jaguar engines where you have a high carbon steel stud fitted to a cast block.