I'm building non load bearing walls for a basement finish project. I'm building them on the floor and raising them up. For the walls that run parallel to the joists above, how far apart should I install blocking to attach the top plate?

That blocking is not too critical, every 3-4' should be enough.

Thanks, is using scrap 2x4 sufficient?

That is fine but you will also need some shim material to tighten it up while you start nailing. I'd recommend pressure treated lumber for the floor plate.

Thanks, already have shims, all floor plates are pressure treated.

Sounds like you are good to go, plumb and square before you drive that first nail, I prefer screws but to each his own.

I agree, screws would seem to be a lot easier than trying to nail overhead; I'm using a Ramset to attached the bottom plate to the concrete floor.