I mix it up. Sometimes I do row by row - sometimes I pair the rows. A lot depends whether my pattern is persnickety or random. With exact placement, I sometimes mark my rows to make sure that I don't do the upside down plan and then have to rip. (Done that many of times).
One thing that is important to me: I pay close attention to the pressing. It helps when you can nest the blocks and that depends on pressing the seams to accommodatet that.