How hardcore to go for prewriting secondaries? Advice?

I'm finally beginning to recover from the stress of submitting my primary app about a week ago. I have seen that some people have started pre-writing secondaries, does anyone who has been through it have advice on how hardcore should we be going? I have heard that it is good to prewrite for schools that you are especially interested in, but it is a good idea to try to prewrite nearly all of them? Would like to hear what others are doing too