Most my shirts have a seam that is straight most the time. Get good lighting and you'll probably see it.
It used to take me over an hour to load and print 50 shirts, a year and a half later and I can do 230 with no loss in accuracy.
Don't go too overboard with spray tak. Only spray it on the print area and use it lightly. Reapply more often, so you get consistent loads.
Pull shirt all the way down to the shoulders, pull back by the shoulder seams to ensure it's straight on the bottom hem. Pull back down evenly by the sides and then pull back by the shoulders to your print position. Check arm seams are equal and side seams (if the shirt is not a tube) at the bottom.
As you get better, you can just load to the shoulders, position and coach sleeve seams.