It will depend on the photos in your libraries, if you want to merge them before the migration.
If the libraries are having all different photos and you do not need to use the photos together in a project like a slideshow or a book etc, you do not need to merge the libraries.
One library in Photos will be special, your System Photos Library. You can only have one System Photos Library. That is the library you can access in the Media Browser from other applications, if you want to share photos, Mail them, etc. And only the System Photos Library can sync with iCloud Photos or Shared albums.
You should merge all your Favorites and the photos you want be able to access from other applications into your System Photos Library. See: System Photo Library overview for Photos on Mac – Apple Support (UK)
Otherwise it helps to keep the Photos Library small. Smaller Libraries are easier to repair and to upgrade.
Photos is needing a lot of processing time to prepare the Photos Libraries, so the size of the library matters more than in iPhoto.