I agree with Carolyn. Keep the collection together.
When you break up a collection you could end up with problems. When you start a new group (because you just received a bunch of similar photos), it can be cumbersome to find previously received similar un-categorized photos and move them into the new group to keep all the similar photos together.
Another problem with grouping (by subject), especially if your cataloging is in bad shape, is, if you receive a photo of “Mary Black” standing in front of the Old General Store, do you put the photo in the “Mary Black” group, or in the “Old General Store” group?