I just figured if they call me Dad it would be very difficult for me to prove them wrong...
I think regardless of what I might think it's still a buzz for me when they do call me 'Dad' (among other things) even now the older ones are in their 40's.
With regard to my particular step-children I never compelled them to do anything except draw their own conclusions from the open and honest truth about their blood heritage, they knew who their biological Father was and they knew they were loved, how much and by whom you cannot BS kids on that level as somewhere inside they all know the truth.
Have I been a succesful parent though? The jury is always out on that, all I know I've never made a distinction between any of them on grounds of paternity although I've always tried dealing with them all as complete individuals.
They have all just been my kids, blessings in my life all of 'em.
Biology does matter to a certain extent as well as it's important to inform their medical practioners with as much relevant information as you can about their blood history as it's unlikely they will develop the same horns on their forehead as I have for example so they won't have to worry about those.
I guess what I'm trying to say is that I've never assumed an entitlement to be called 'Dad' by my stepchildren but it's a complete buzz and a priviledge that they do call me that and want to stay in touch.