It's totally dependent on the recipe and the results in terms of ABV% and the desired flavour balance. Generally, I don't dilute for my own consumption (and that of my highly wonju experienced circle of friends ;) ) but certainly when we do public events or if I'm giving bottles to my extended Korean family I will dilute. That being said, every batch comes out differently, and not all lend themselves to dilution.
A sour-heavy batch when diluted will open up the sour notes even more and does not fare well with say a 50-50 dilution (which in theory would bring a high ABV wonju down to the usual commercial standard percentage.) I find that if I'm making a batch intended for dilution, I will aim for a sweeter overall profile so that when water is added it still has some flavour and character.
One of my favourite commercial makgeolli's here in Korea has a 6, 9, 12 and 18% makgeolli (the 18% being really just wonju), and while I normally love and prefer high ABV sool, it's his 6% that is the shining star because it has a balance of all elements. The 18% for me is just way too sweet and heavy-bodied.....but he makes it like that so after dilution, he can have the perfect sweet spot for his 6%.
What I tend to do if I'm diluting is to add the water gradually and continually taste. I start at about 20% and then add in smaller increments, continually tasting until I'm happy with where it's at. If it happens to be losing too much flavour, I'll sometime beef it up with a fermented syrup or some fruit, and then age it in the fridge for another 1 - 2 weeks. I usually don't like to dilute and serve immediately, as it needs time to blend.