do i need to keep my old iPhoto library after migrating to photos

No. If you are certain that you won't want to use iPhoto again you can delete the library. It won't free up a lot of space but you can safely delete it. See the following re the space: Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Need to - no - deleting it has no effect on Photos

HOWEVER

It also saves very little and if by chance you want or need it in the future then you will be in a world of hurt so it is strongly recommended that you do not delete the iPHtoo library for several months

LN