Most of the time, yes. However, for higher-overclocked GPU's, the vendors tend to bin or sort through their supply of GPU's to find chips that can handle the higher clocks. I know that EVGA did this for their SSC and FTW series of graphics cards and Gigabyte does it for their Super Overclock Series.
A lot of people find it a waste of money to pay more for a card that's been overclocked only a couple MHz that you could do yourself with a reference-design cards. It's really up to you whether you want to pay more for a factory-overclocked card.