As swsa says below, the major shift comes in 'Touched'. Of course Buffy *does* lean on Spike greatly throughout S6 and S7 (early S6 esp shows Buffy letting Spike 'be there for her' the way Riley wanted to), but Touched is the great divider. She lets Spike in, acknowledging that she needs him ("Will you just... hold me?") and later thanking him for giving her strength. It's exactly what Riley asked for and never got. She lets Spike be close.