MacBook turned off during Time Machine initial back up - What do I do?
It would be hard to tell with out looking at the backup. If I was in you situation and I was not sure if the first backup was successful I would wipe that partition and start again (assuming when you say initial backup you mean the first ever!)

From my personal experience however when similar things have happened to me during a backup I've had no corruption issues. This being said it was not my first ever Time Machine backup.

Honestly the safest thing to do is wipe that partition and start again (assuming when you say initial backup you mean the first ever!)