Do I need to back up my MacBook Pro if I need to replace my keyboard?

Apple assumes the user has good verified working backups so Apple will erase or replace the drive if Apple deems it necessary during the repair without contacting the user first unless it affects the repair cost to you. An AASP may offer to backup your system if it becomes necessary to erase or replace the drive, but it would cost you extra and this varies from one AASP to another.

@BobTheFisherman is correct that every system should have good verified working backups all the time since you never know what might happen to your data while using your computer.

You need to backup your data often and frequently and before every major repair.