Transferring photo libraries? I have two macs, one with Mountain Lion and one with Mavericks. Do I need to put Mavericks on the other computer to be able to properly transfer the photo libraries?
The version of your operating system is not important here. You need to be careful of the version of your iPhoto. 

Old iPhoto library can be updated by newer iPhotos. But newer iPhoto library cannot be opened by old iPhotos. So if your target mac has an up-to-date iPhoto, you can copy it directly.