There's no specific percentage given to step 1 or step 2 when it comes to matching. Both scores are important and both will be used to assess applicants and give preferences.
However, USMLE Step 1 score may have more weight in the assessment than Step 2 for the following two reasons:
Early in the match (September to December) most of the American students apply for the match while they don't have Step 2 CK score ready yet. So they compete with you in their Step 1 score only. Therefore, program directors will be looking more at Step 1 as it is the only common denominator among all applicants US and IMG grads.
Step 1 is generally perceived as the more difficult step and therefore program directors will highly appreciate a high score in it as opposed to CK score which most of students get high scores in it.
StepTaker is right, but.... Ultimately, while no program will make an explicit rule about scores, Step 1 is easily the most important step score. Why? Just think about it logically: there are thousands of applicants from hundreds of schools. Programs need to quickly compare them in order to determine which 100-200 to extend interview invitations to. The quickest comparison factor? USMLE Step 1 Score. You have to have a good USMLE Step 1 study strategy in order to excel on the test. As they say, the early bird gets the worm. So, start planning ahead years earlier if you want a US residency post. Study the First Aid for USMLE books. Take qbanks online, whether they are Kaplan or USMLE World or some other source. Basically:
* Prepare early; plan ahead
* Practice makes perfect, so practice hard for Step 1
* Just do it - make sure to review your mistakes so you never make them again
The key is balance - don't overstress or understress. Be confident and take care of today - tomorrow will take care of itself!