If you're working with a small-displacement engine, it can be tough to achieve compression ratios even as high as 10:0:1. If you're working for every little point of compression you can get, it can be beneficial to order your pistons with no valve reliefs, then have a machine shop cut them only as deep as what's really required for ample valve-to-piston clearance with your combo. No need to give up valuable compression with valve reliefs that are way deeper than what's needed.