That all depends on how bad things look in there. If there's rust, buildup, sludge, deposits, scale or the drained coolant looks like it was severely neglected, a chemical coolant flush additive might help. That said, I've heard most coolant flush products aren't terribly effective these days due to an EPA crackdown in recent years.
It's not really a good idea to flush with plain water, as you inevitably will end up with at least some of it still in the system when you're done. Distilled is cheap enough to use and you negate the risk of introducing minerals, salt from water softening systems, chlorine, etc, that you really don't want in there.
