OP do what you want, if you want to pray to the sky all day join the old men and if you want to go dragon hunting with a group of friends join the blades. I killed Paarthurnax, but the old men still remained friendly to me. UESP says it has something to do we me having found all the the non-questlocked/non-DLC words of power BEFORE I killed Paarth, so I got lucky.
Just make up your own mind OP, nothing much changes.
I agree with the do what you want OP part, but I don't agree that sparing Paarthurnax means you have to join the Greybeards, or do the things they do. I spared him, and I still go and kill tons of stuff, use my voice to destroy my foes, and go hunt dragons with my friends. Not killing Paarth doesn't make you a Greybeard, IMO.