Flycutting is not specific to machining pistons....it's a machining process, using a certain type of tool. You can fly cut anything. If you read the original post, he's asking about cutting his heads, not his pistons.
As Mike said, valve reliefs in pistons have everything to do with piston to valve clearance due to agressive cam profiles and timing, decreased deck clearance, or even "pop-up" or domed pistons. Compression itself has nothing to do with it. Some of the things that people do to try to increase compression may result in decreased PTV clearance, such as a decked block, agressively machined heads to decrease combustion chamber volume, or dome pistons. But, you can run a long stroke motor that has 12.5:1 compression with flat top pistons and stock heads and not need valve reliefs in the pistons depending on your cam setup. Or, alternatively, you can run an engine with a low compression ratio, and require valve reliefs because of a very aggressive cam profile, or advanced cam timing.
Lazerviking - Just cut them enough to ensure flatness. Find a local speed shop. Or, some NAPAs have machine shops.