In case of ntuples, it is not even necessary to generate histograms at all. Most of the time, you just want to see the cloud of 3-D points with some associated color, marker shape or size depending on a 4th or 5th or 6th dimension. This has the advantage that points can be drawn precisely with their original x,y,z values and not approximated if they come from a 3-D cell where they have been accumulated. This feature may be particularly interesting to minimize the total number of cells required and optimize the resolution for some areas. In my view, 3-D histograms or higher are justified only in case the time to process the original data base is prohibitive.
Garbage collection is automatic for all objects generated on the fly.