You said that the modern browsers can resize px font size values but IE6 and something below can not. So what do you think? In my opinion I think that px values are okay for your own website, but if you create something for business I think the better way is to use em instead of px.
For my opinion I refer to the W3C Counter Global Stats. Okay IE 7 is also widely spreaded but I think its not a big problem to use ems.