For the majority of the flies I tie, I rarely use head cement on my whip finish. But if I am tying a saltwater fly or a larger streamer type pattern, I do like to put a drop of my favorite glue, Liquid Fusion, on to help increase the durability. Most people put the head cement on after they have finished the fly with a whip finish. I like to add it before I tie the knot. I will put a drop on the head before my last few thread wraps. I then do thread wraps over the dollop of glue, spreading it throughout the entire head of the fly, before I whip finish. This gets the glue inside the knot instead of just on the surface. I believe it helps even further increase the durability of the fly. Happy tying!
Previous Tips:
Interesting and will have to try to Liquid Fusion! Been transitioning to UV resins for most of my fly finishes, but the added cure time adds up when in production mode!!