I'd say keep them intact. Singles aren't that tough to find. The sets themselves don't have enough high demand singles to justify it. A Maddux just sold for $3.02 in PSA 9, ungraded copies often go in the $1-2 range. After you sell the Maddux, and maybe the McGwire, you've got a handful of other cards that don't carry much premium over the base versions. The 1988 set might do okay as singles, but I wouldn't figure it'd do well enough to justify the extra auction fees.