I have an electric fan, which was installed by the guy who did the restoration. He lived in the desert near Sacramento and wired it so it was on whenever the ignition was on. It turned out to be a little too much cooling on cool days here near the coast--the engine never quite got really warm. So, I added an electronic thermostat. Now, it hardly ever goes on, and the car stays well below 180 most of the time, even with the stopped fan blocking the radiator a bit. On really hot days, in traffic, it will creep up to a level of borderline concern--like 190 or a bit above. Then the fan goes on.
Forced air cooling is surprisingly efficient. And, yes, the stock system should be fine if everything is right--but things, especially in these cars, don't always work perfectly. A fan gives you a bit of margin for that imperfection.
On the subject of imperfection, keep in mind that your temp gauge is probably not better than about plus or minus ten degrees. Also, the temperature of the water leaving the engine is quite a bit higher (~ 10 degrees) than at the gauge bulb. (I measured these temps with a thermocouple sensor in order to get the fan thermostat right; it's quite an education.)
If you do add a fan, which, frankly, I think may be worthwhile, don't just get it from the Usual Suspects, unless you like paying double what it should cost. Get it from your local Pep Boys or some similar place. Last time I was there, they had some nice ones for good prices. Electronic stores often have nice ones, too.