TIPS FOR CUTTING FABRIC + PIECING WITH TEMPLATES:
I think the most common mistake when piecing with templates is not printing the templates at the correct size. Before you print the templates double check your printer settings. In the program I use (Foxit Reader), I can adjust the scale that the image is printed out at (most other programs have this option as well). I select the "Custom Scale" option and make sure the percentage is at 100% (A screenshot of my printer settings is below).
Another option you might see in the program you print from is "Do Not Scale". If you select that it will also print the template at full size. Print out your template page and now, using a ruler, check that the 1" test square box on the page is EXACTLY 1" square. If it's even a little bit off, that discrepancy will add up and result in your block or unit not finishing the correct size.