Question: we know that view state is easily get abused, but asp.net webform are heavily depending on this feature.

I want to know whether you disable viewstate by default, and ONLY add it wehn it's needed. Or you take the visual studio default, which actually enables viewstate by default.

Answer: 
I tend to take the default of having it on when doing web.forms. 

But when writing my own user controls I disable it until I need it, or I find something doesn't work. I also monitor the size of the view state and when/if it gets too big I look at what the page is doing and change what I'm doing on the page. I.e. bind to data objects containing only the data that is needed and no more... stuff like that.
Personnaly, no. Eventually, to have it disabled will be the behaviour that you will want but most of the time, no. Also, I would do a manual ViewState optimization pass and disabled controls' ViewState that wouldn't need it if really necessary. It's better not to have the ViewState to worry about while you are in heavy development imo. I'm not saying here that you shouldn't care about the ViewState at all but to put EnableViewState aside until you feel the need to lighten the trips to the server.