Most people these days sign up for websites at where they live rather than on public/work computers. So, on forms that require the user to enter their address, should they be filled in by default with their current location?
That's a big assumption you are making in there. I would say that it's the very opposite because of all the smart devices. User can be anywhere and by filling the form with false data sounds really confusing and dangerous. Keep it simple, don't try to be too smart. :)
Never - People are mobile and so are devices theirfore the application would pick up on the current location based upon the wifi/location sensor in the device. Also, VPN's (which I use all the time) can potentially places the user outside of the location they care to receive packages.
I'd say this is overkill, but if you really want to be smart, maybe you could get the user's current location behind the scenes, and when he/she starts filling in the form, auto-suggest the current location if it seems to correspond.
No, this shouldn't be default. You could offer it as an option but I wouldn't give the user too much choices because of reasons already mentioned (false data). And not ever user is happy when websites try to get their location automatically because of privacy concerns - although they can turn location off, of course.
I would only offer this option if the location is really helpful for executing the users' task (eg. travel planners, public transport planners).