"Headwork" covers quite a wide range of work; from polishing the ports a bit, through to gas-flow work and fitting enlarged valves.
If you mean just polishing the ports yourself, then go ahead! Won't make much difference but it won't hurt either and it's a good experience to have a go yourself.
If you mean professional porting / gas flowing then it's about value for money. A well flowed head is always beneficial, but it becomes more beneficial the more power you are aiming for. Both the 16VT and 20VT can make well over 400 bhp without any headwork. However, if you are aiming for figures above 500 bhp then headwork begins to become essential.