Hi Phillyfly :),
Great question and interesting topic. I am with you in wanting to drink only when thirsty, both when fasting and otherwise. When not fasting i find myself almost never drinking water, getting ample throughout each and every day from the vegetables and fruits i eat including soups and smoothies which contain a certain amount of added distilled water in addition to what is already there.
I have done a bit of research on thirst and also on "dry fasting" and have found the literature to be rather scant and elusive. I believe i read somewhere that actual thirst is represented in the throat where it will persist and not merely in the mouth where it will come and go. When i am fasting i await this moderate thirst that i feel in the throat and will generally resist water comfortably each day for about 15-20 hours (including sleep) before satisfying thirst with distilled water.
Some time shortly after this moderate persisting thirst has been reached and if abstinence from water was to persist, first stage dehydration would begin to manifest itself and would generally include drying skin. In wanting to "drink only to thirst" and in experimenting my way through it about a year ago, i went beyond moderate thirst one time in a 40 hour dry fast in the heat of summer, in a dry climate and with a lot of brisk walking. In this one instance i allowed mild drying skin to arrive and this has helped to shape my ideas and practices, opting to avert any such symptomology each time to follow. Resisting mild thirst for about 15-20 hours many times thereafter and awaiting a moderate and persisting thirst before drinking has never (for me) been met with any preliminary symptoms of dehydration.
Some Curezone threads on dry fasting which include posts by the knowledgable and experienced Fonty and a little bit of feedback by Chrisb1:
http://curezone.com/forums/am.asp?i=1155596
http://curezone.com/forums/am.asp?i=1008243
http://curezone.com/forums/am.asp?i=1135470