I have always used hand hot water all round the car .. Not boiling :(
Between warm and hot :D
I've used this method for the last 20 years and never had a screen crack on me ..
I would however say, that if you already have cracks or major chips in your screen ? Not to use this method.. I don't know for sure it would worsen the crack or chip ? But logic says theres a good possibility it would ..
If you do use this method, do not use the water sparingly.. A whole kettle sized jug should be used for the front screen.. This is so the you do not just melt the ice, but also raise the temperature of the screen high enough so as the screen does not re-freeze.. Wipe of the excess with your wipers once all the water has been used .