It's all personal preference I would believe. I am not sure, but I am guessing that if your iPhone is using wifi rather than 3g to transfer data, it is not really using that much power since you aren't actually using 3g. But I am not 100% sure.
It would probably take more battery disabling the 3g than you'd save by switching it off. Not to mention the hassle involved in that! You'd have to be fairly patient to be arsed with that.
For what it's worth, if I'm in uni all day, I can be connected to wi-fi from 9am until 5.30pm and, with 3g on and minimal actual usage of the phone, I'll still come home with a full battery indicator*.
So it's certainly not a deal breaker.
* In contrast to what I said in another thread about my crappy battery life! It appeared to sort itself out after a few days of owning it.
I have WiFi at home and at work. So when I am at either place it automatically connects to the network using WiFi. All the other times it is on 3G. I don't even think about it, it just connects to either one. I think switching 3G off and on is an added step that is really not necessary. I have never had any battery issues. I just dock the phone at night, and the next morning it is good to go for the day.
How strong is your 3G connection? If it's weak or constantly switching back and forth from EDGE, this might make sense. Otherwise, I can't imagine the hassle being justified by the marginal increase in battery life.
For what it's worth, if I'm in uni all day, I can be connected to wi-fi from 9am until 5.30pm and, with 3g on and minimal actual usage of the phone, I'll still come home with a full battery indicator*.