Gotta say, I think the episode Selfless did a lot to change his attitude towards Spike. I just love watching that argument - with Buffy and him getting more and more vehement and angry, and then Buffy says "I killed Angel!" (and FINALLY brings up Xander's lie), and he just seems so incredibly shocked.
After that, about the closest he ever gets to questioning Buffy's decisions is "Buff, I'm trying to see your point. But I think it must be to my left," in Empty Places.