The truth is, you do not have to be certified in anything to land most welding jobs, even on the North Slope. You have to be "qualified", and they will do that on-site, or at a lower-48 test facility they've approved. You see, you will be welding to their particular WPS(s) that may or may not have been covered in your certification test, and you will have to test to that/those standards no matter how many certifications you carry. They will want to see your product for themselves, rather than a piece of paper.
That said, having a certification or two to put on your resume' tells them you have test-proven skills and are worth the expense of testing for the qualification. The more skills you can combine (TIG root/7018 out, 6010 root/7010 out, etc.), the more marketable you are. Don't invest a fortune in certs. Maybe choose two that are diverse, to show multiple talents. Pick the ones you enjoy most and seek the jobs that require them and you'll be happier.