I've done it both ways in the past. I'm not sure what you mean by [e]conservative[/e], but I've found that most of the time the standard calculation results in a larger service. I haven't done any of these calculations, with the exception of my own house, in years. I used the standard calculation on my house. It seems like there were a few times when the optional method resulted in a larger service, but not very often.
The Watt Doctor