We've seen many members asking the same question repeatedly. Therefore, I thought let me give some facts about this.
We are talking here about not taking the exam even after extending the eligibility period, i.e., you signed up for the first three months eligibility period and then you extended to another three months and yet you did not take the exam.
Here's the deal:
If you don't go to the Prometric Center, you are not counted as having had a USMLE step attempt
regardless of whether you officially cancelled your date with Prometric and/or ECFMG and/or NBME.
Exam attempts are considered only when you actually go to the Prometric center and register for the exam.
If you are still skeptical about what am saying, here's an excerpt from the official USMLE Bulletin
Q) Would cancelling affect my residency matching profile?
Except as otherwise specified below, to receive a score on Step 1, Step 2 CK, and Step 3, you must begin every block of the test. If you do not begin every block, no results are reported, and the "incomplete examination" attempt appears on your USMLE transcript. If you register for but do not begin an examination, no record of the test will appear on your transcript.
A) No. It won't be reported and it does not affect your chances.
Q) Would it be considered incomplete attempt?
A) No. Only if you go to Prometric center and register and start the exam and then you leave mid exam, only then it's considered incomplete attempt.
Q) Would incomplete attempts bad for my record?
A) Yes. It's considered a failed attempt.
Q) What are the disadvantages of cancelling a USMLE Exam then?
A) You lose the money. You have to pay the fees again. That's the only disadvantage.
Note that what's mentioned above still holds true even in the face of the new rules of limited attempts and minimum spacing posted in the ECFMG website respectively here