How early is too early to start thinking about SMPs/Post-Baccs?

For example, could a freshman do anything to be able to "already know" they're going to have to do a Post Bacc? Wouldn't it be ridiculous to do very well for the next 3 years but in order for medical schools to forgive 1 bad year you need that extra year of post-bacc?
Source: Student Doctor Network - Category: Universities & Medical Training Authors: Source Type: forums