Well, so far, it looks like a rough 50-50 split between those who wouldn't dare touch their negatives (OP included) and those that either would, could, or sometimes do, although evidently very rarely for purely aesthetic purposes. Evidently, most who do manually 'retouch' their negatives are rectifying basic anomalies in the emulsion rather than removing wrinkles, softening rough skin, removing unwanted thumbs upsetting the composition, etc.
Frankly, that is more than I anticipated.
Very, very interesting to know. Thanks to all who responded.