Final answer:
Not heat-fixing is specifically prohibited in the acid-fast staining technique, as heat is necessary to ensure that the dye penetrates the waxy cell wall of certain bacteria.
Step-by-step explanation:
Not heat-fixing is prohibited in the acid-fast staining procedure, which distinguishes bacteria with waxy cell walls by retaining the primary stain even after the application of a decolorizing agent. This integral step involves the use of heat to facilitate the absorption of the dye by cells, particularly for identifying Mycobacterium species. In contrast, procedures like endospore staining and Gram staining do typically include a heat-fixing step.