when strip piecing it is best to alternate the direction with each new strip- as in- you start at one end for the first strip added then start at the end you finished on for the next strip- this keeps your strip set from curving/bowing as it grows- the best way to do this is to either always have the new strip on top- or always have it on the bottom- which ever is easier for you- there is no right or wrong- everyone does it the way it's comfortable for them- the only thing important is alternating those ends...the longer the strips the more pronounced the curve can be if you always start at the same end.