I would also wash either all or nothing. I also wash my pre cuts. You can swish them in the sink with hot water & a couple drops of laundry detergent. then rinse. If you have a salad spinner you can use that to get rid of excess water if not, just press between towels. As mentioned, you can check your local grocery store for starch in the laundry detergent section. I know other Canadian members have mentioned that it's hard to find. It's usually less expensive than the best press and will result in a stiffer product.
I prefer the ruler without the tips cut off.
One side of your triangle will be on the straight of grain. Depending on how you want your fabric print to go, you may be able to place a straight of grain edge against a bias edge on at least some of your pieces.