Is it required to report climbs and descents within a block altitude assignment?
When given a CRUISE clearance [...] where does it say you don't have to report altitude changes.

In my opinion, it say that by implication in the text you quoted, emphasis mine:

The pilot may level off at any intermediate altitude within this block of airspace. Climb/descent within the block is to be made at the discretion of the pilot. However, once the pilot starts descent and verbally reports leaving an altitude in the block, the pilot may not return to that altitude without additional ATC clearance.

I read that to mean, and as a controller I was taught that that means, you can climb and descend to your heart's content while operating on a cruise clearance. Only once you report leaving an altitude are you required to remain below that altitude; therefore by implication you are allowed to return to that altitude so long as you don't report leaving it, and therefore by further implication you are not required to so report in the first place.
The AIM references the Pilot/Controller Glossary term &quot;cruise&quot; which contains the same exact verbiage.
The 7110.65 contains instructions to controllers that further this interpretation, particularly 6–6–2c under the Non-Radar section and 5–6–6a under the Radar section, which say that controllers cannot use Mode C altitudes to guarantee vertical separation if one of the aircraft involved is on a cruise clearance—because the aircraft is allowed to climb or descend without notice.