yeah, I would wait. A name isn't what gives the attachment. There is also a good chance their bio-dad will be in and out of the picture a ton anyhow so they may end up with negative feelings towards "dad."
My boys call their step-mom "mommy." Drives me insane but that is what they were taught. She's a good step-mom so in that way Im grateful. But, now her son with my ex is starting to call me "mommy" so I think she understands how confusing it is for kids to have to figure out what to call everyone in their life.
Suzi, working mama to my ODS(2004)
and married to the love of my life