My experience was that it was pointless to exchange for peso's. Everyone seems to be flexible in what their prices are and I just used ones to tip everyone. But be careful as many shop owners will try to really do bad math on converting. But like I said barter and walk if they don't like what you have to offer as chances are there is more right next door. The only Pesos I got were ones that came home with me for the kids. Everything there though was usually listed in both pesos and US Dollars.
Good advice.