Do them one wheel at a time, then you will be sure all air and old fluid is removed completely between the Master cylinder and that wheel cylinder. Otherwise unequal lengths in the lines, potentialy unequal pressures, you may leave some old fluid or air in your lines, probably the farthest back one. Finally, you can better keep up with the fluid level in your master cylinder making sure it does not run dry and pump more air into your lines goind with one at a time.
