If you look at the geometry involved I think you'll see it's not critical that your cut be perfectly perpendicular to the fibers...
I would agree to not agonize over it. But I'm always surprised by how many times I go to restaurants and get brisket that's sliced almost exactly with the grain, so that tells me that even people working in professional kitchens can't figure out how to slice a brisket across the grain! "Marking" the brisket just takes the guess work out of it, if it's something you care about. Much more important in competition than in the backyard.