I\'ve found it depends on the climate in your area
After talking with some of my out of state friends, We\'ve noticed it depends on the climate of where you are. A friend in on the east coast uses flow improver in the summer since its hot and humid which makes the paint clump, but he\'s fine in the winter with just water, a friend in Vegas uses retardant since it\'s really dry out there (and maybe a little flow imp. depending on the weather) with his water. I\'m in Southern California, so I use a mix of all three in different ratios depending on what it\'s like out side or how far I am from the beach when I paint (the climate changes about every 5 miles you go inland).