Actually, your method may put the film on the covering frame with a more "natural" amount of tension in the X vs. Y direction.
I adjust the tension *after* putting the film on the covering frame. The covering frame rails are coated with Vaseline to allow tension adjustment. However, I've often found that I put too much tension in the fore-aft direction.
My problem may also be due the covering frame having only two sides instead of four. In order to remove wrinkles from the ends, the film in the middle has to be pulled very tight.