Does a creature that deals damage via deathtouch need to deal lethal damage to each blocker before assigning to the next?

Normally, an attacking creature must assign lethal damage to each blocking creature before it can assign any damage to the next blocker in the damage assignment order (CR 510.1c). Lethal damage is generally defined as enough damage to equal or exceed the blocker's toughness, accounting for any damage already marked on it.

However, CR 510.1d creates a special exception for deathtouch: any amount of damage dealt by a source with deathtouch is considered lethal damage. This means a creature with deathtouch satisfies the 'lethal damage assigned' requirement by assigning just 1 damage to a blocker, even if that blocker has a much higher toughness.

The practical result is that a creature with deathtouch can 'trample over' multiple blockers efficiently, or spread its damage across many blockers, by assigning only 1 damage to each one before moving on. This interacts powerfully with trample, since the attacker can assign minimum lethal damage (1 per blocker) and then assign the remainder to the defending player.

Example: A 6/6 creature with deathtouch attacks and is blocked by three 5/5 creatures. In the damage assignment step, the attacker's controller may assign 1 damage to the first blocker, 1 to the second, 1 to the third, and 3 to nothing (or, if it also had trample, 3 to the defending player). All three blockers will be destroyed at end of combat because any damage from a deathtouch source is lethal (CR 702.2b).

So the answer depends on whether the creature has deathtouch: without it, full lethal damage must be assigned to each blocker first; with it, just 1 damage per blocker satisfies the requirement.