Yes, it's a good idea to add an extra third to the development time of every project because a tiny, tiny number of people turn off their javascript. And don't forget that some people surf the web with their eyes closed and some do it by simply shouting at the screen, so you'll have to accommodate them too. And what about people who don't own computers at all? You absolutely have to make provisions for them too.
With a bit of effort and by listening to enough people on this forum you can increase every project's cost by a factor of 100 and win every pennyless knuckle-dragging idiot on earth as a customer.