Yes starch really really helps( I do this prior to cutting) ... and I do press my seams open rather than to one side.. When you get to the final tube seam... do not use the raw edges of the first and last strip to line up for that last seam. Lay the strip set down( prior to the last seam) and line up the seams at the ends disregard where you started to sew the seams as your guide or the ends where you will make your colum cuts ( at this point) ... its highly unlikely they will line up. Once you have the seams lined up and everything is laying flat.... then pin and sew. Trim off the wonky part of the beginning of the strip set.
I use 1/2 width cuts of the fabric for my bargellos... so much easier to work with escpecially when it comes to getting that last seam .. and trying to line it all up and lay flat. Full lenght strip sets always will get some bit twist ( for me at least).