"The save RAM controller could also be designed to handle a power-down at any time, and perhaps the MMC3 and later do this (I don't know if they do)."
Nintendo Power, in its late-1990 feature titled something to the effect of "Why Game Paks Never Forget", mentioned that the MMC5 had an improved circuit for SRAM protection on power down. Perhaps the MMC6 (if I recall correctly, MMC6 is an MMC3 with a built-in SRAM) has the improved circuit as well.