It depends on what the particular school wants. Many schools have particular requirements about LOR's. Some schools want a certain number and may only take the first three you send, for example. Some have specific requirements, like two science faculty and one non-science, or 2 science faculty and one from someone who can verify your patient experience. Some may not take them at if they haven't sent you a secondary. Many have preferred forms that they want your writer's to fill out, but most will accept a letter on their professional letterhead instead.
You should start now letting people know you want them to write LOR's because some of them take a long time to do it...but you need to find out the school's policy first. You might want to check into Interfolio if the schools you want to apply to accept it. It makes it easier sometimes if you are applying to a lot of schools.
Personally I wouldnt do it. I waited to hear from the schools first and then sent them. Especially the schools that screen their secondaries.
Good luck.
I think I'm just gonna send my secondary into VCOM before they tell me I can fill it out and then just show up for the interview and hope for the best. ha ha ha