Basically, you should contact them whenever you are ready. Your US business roadmap provides you with a timeline for each step and each module to help you assess when it is best for you to contact Sherpas; you then decide. Some companies will want to talk to Sherpas very early in the process to make sure they have everything under control and to get organized ahead of time; other companies will rather decide to contact them as they progress through their US business roadmap.
Since you will have to digest a lot of information as you start implementing your US project (and to connect many dots!), we recommend that you do not wait until the last minute, of course.