When to sign up for visitor medical insurance?

Since all the online visitor medical insurance take effect as soon as we click on the web, is it better to not take the insurance in advance and save some money? We need to take the insurance only at the time of need if need be. Please advise if I'm missing something..

Well , your quetion is genuine BUT

Don't forget about the "Pre Exisiting condition clause" , as long as its general illness it OK, but if the Dr. determines that the problem is Since .......??

Also how about the emergency ? will you be able runing to get on the NET and buying the insurance or taking care of the situation??