When you started with OS X Photos it will create a new library (Photos Library.photoslibrary) and migrate your old iPhoto library (iPhoto Library.migratedphotolibrary). So when you are fine with using only Photos you can safely delete it iPhoto and the iPhoto library to free up some harddisk space. To be save I moved the iPhoto Library.migratedphotolibrary to an external drive.
I noticed that they both showed nearly the same size but when you delete the old iPhoto library it will only free up some of that HD space and not the total Size taken by the iPhoto library. My photo libraries were about 220GB and after deletion of the iPhoto library I freed up about 30GB, which is still a significant free space on my HD :-)
Also see here: https://discussions.apple.com/message/28114501#28114501 where they note that it will save little space, still I like all the space available, especially when I don't use iPhoto anymore ;)