Am almost complete with a gut and remodel of a family room. I have all the drywall taped, with the exception of the inside corners where the ceiling meets the walls.
I hung the drywall horizontal, so on two walls the factory tapered edges on both the ceiling and wall meet up. Should I tape the same way I'd tape any other inside angle? Seems it will be hard to get a nice flat 90 degree corner with both edges tapered- and also won't have anything solid to run the knife along while taping. Should I pre-fill these corners so they are flatter before I tape?
No! Just tape them as you did with all the other inside corners. As you give it the 2nd coat you will automatically fill the depression. It's easier to do one side at a time on the second coat. Otherwise, you will dig out the 1st side when you apply the 2nd side.