Perfectly possible to make a good head without flow testing each individual one, once you know what works its just a case of replicating it, and the best way to find what works is to test it on an engine.

flow test data is useful, but its also not directly relevant on its own, if you compare 2 heads for sale and go for the one with the biggest flow value, it tells you nothing about how it works on an engine just by looking at that figure in isolation.

like garage19 says, flow benches are a useful development tool to get max flow from min size (essentially the desired end result), but ultimately if you want to make a head with a big flow figure to sell it then you just make the hole bigger!

I have no idea what the flowbench value for my cylinder head would be, but I do know that it works very well on my engine.