You'll definitely need some sort of filtering/smoothing particularly if only using a single diode to rectify. Even with a full wave it will drop to zero every half cycle:



So enough capacitance to ride through that drop every 16ms (assuming it's 60Hz). You can fairly accurately size it. Probably won't have to be that large since the peak voltage is much higher than what will be the minimum input voltage to the buck for your 5V.

So it's 24VAC *RMS*? And you need 150mA at 5V?