it seems it depends on firmware you use determines which you use, had a good look on diyautotune site for installing to ford zetec and for some it's rising edge and some it's falling edge.
i'm using code 2.1.1 extra code and that runs best with falling edge, where as on ms regular B&G code then on using edis you need to use rising edge.

i had no idea about this till this post popped it's head up again.