iPhones, as well as every other Apple product that uses a battery has smart circuitry built into them. The way a battery is considered full depends on the chemical.
For example when charging a NiMH battery when it gets full there's a drop in the amount of voltage it uses. That's how battery chargers know when to stop charging them. Now if you take that battery out and then immediately put it back in, you could risk that voltage spike never showing since it's already fully charged, and you could then overcharge the battery.
LIon batteries that all the i* products use aren't as smart as that, which is actually good. I honestly don't remember how you tell when one is fully charged. iPods and now iPhones are known for their battery circuitry being too smart for it's own good. That's why you'll see the battery at 50%, plug it in and it immediately jumps to 75%, unplug it and it's at 60%, etc. It's trying to guess how much power you have left. Although you don't have to "re-cycle" a LIon battery for charging, you do have to go through a full discharge/recharge cycle in order for the charging circuitry to learn how the battery charges and to get an accurate guess at how much power is left.
I appeared to have horrible battery life until I tought the iphone. Basically I used it until it was at least 50% or so, then I would charge it over night. In the morning I leave it plugged into the power, turn it off and back on, and let it stay plugged in until it said it was fully charged again. Did this a few times and now the battery meter seems to be pretty accurate and doesn't jump around as much.
...yeah, getting back to the original question. Nope, you shouldn't have to worry about overcharging it. But I wouldn't keep it "topped off" all the time. It's good for the battery if you actually use it. My typical daily routine is leave it plugged in at work. I then don't bother charging it overnight and then plug it in at work the next day. Basically, only charge it once in a 24 hour period. Don't charge it at work, then plug into charger in car, then plug into charger at home all evening.