I used to put to lengths together making the seam down the middle but that caused a lump when advancing. Then I read that I should put my seams across as that would alleviate the lump so since then that's what I've been doing. Of course I'd rather use backing fabric and alleviate the seam altogether but as mentioned before sometimes you just can't find the perfect backing. Haven't tried the John Flynn method yet.
Also when I do have to put 2 pieces together I glue the seams open so they don't flip on me while advancing. I use Elmer's Glue so it will wash out later. Love Elmer's Glue.