I agree, if you have as5 go ahead and use it. just make sure you get all the stock stuff off ot the sink. I you are lazy just leave it as-is and enjoy the proc. I was lazy on mine and left it on there. runs great w/ a 200mhz oc and no temp diffs from stock speeds.
I've used the stock heat sink and fan on both of my rigs (one in my sig; the other is a 3200+). The sig-rig rarely gets over 42c, and is usually <40c. I have not overclocked. If you are going to overclock by any significant amount, then you probably want a 3rd party cooler anyway. Replacing the existing pad with AS-5 may make a tiny difference, but not enough to be worthwhile (2c-5c maybe). I'm not sure I have any reason to bother replacing my stock cooler unless the fan fails, as it doesn't seem particularly loud.