Potatoes are the one thing that i have been growing for years, and regarding watering, my potatoes flourish when i do this:
I find that deep watering is my best method.
Potatoes can take some periods of dry weather. However, if it's been very dry and the tubers are beginning to develop underground, water the plants. If you need to water, be sure you do a thorough job, applying enough to moisten the soil 8- to 10-inches below ground.
Also, as already mentioned, the potato is underground, so i have always let mine sprawl, with no harm to the potato underneath.