Have you heard the phrase "six of one, half dozen of the other"?
I don't see any major advantage either way. Adding keywords is the same amount of work, whether you do it at import or after import or some combination.
If you already have your keywords entered on some other images and have them set up as hierarchy sub-sets (such as "Place>Location>Specific Location" and so on) then I'd say do your keywording after import. During the import process you have to Type In keywords. After import you can assign keywords from your current list, hierarchy and sub-set, by selecting images and then putting a Check Mark in the specific keyword.
A hint from Ben Willmore (CreativeLive)-
Only keyword your 'Star rated' images- the photos you will want to use in future. Makes the keywording job shorter. (IF you have applied Star ratings!!!)
Rob Hallett wrote:
The folder tree structure will provide at least 2 or 3 keywords for each image in a particular folder.
This script may help automate that - Copy Mac Finder paths to keyword hierarchy