I think everyone's forgetting the part where he is doing a top swap that's why the heads need to be pulled off so why not make it easier for himself just in case he doesn't have a decent sized air compressor to do the valve while they're still on the car. Plus he's having to change the head gasket anyways just save yourself the time (when you do the cam) and go ahead and deal with heads all at once and valve springs are a lot easier to do with the heads off then on especially the rear)