Hidden Machines (HMs) in Black/White
Pokémon Black & Pokémon White actually cut back the number of HMs from previous games, from eight in Generation 4 down to six. None of the HMs are required to complete the main storyline except Cut, which is used on a single tree.
However, they are useful in most areas to access extras like items behind trees or across water. Several large rocks can be moved with Strength to create shortcuts. Flash remains a TM but is useful in certain caves. Headbutt and Rock Smash have no use outside of battle in the Unova region. Dive is only used for one area - the Abyssal Ruins underneath Undella Bay (more details & map).
The Pokémon can be filtered by ticking the checkboxes. This is useful for deciding on an
HM Slave - a Pokémon knowing only HM moves, to save all four spots for good moves on other Pokémon.
Where to find the HMs in Black/White.
Pokémon HM chart