This is something my mum raised only last night! My OH and i decided ages ago, that providing its ok with them, our mum's and nan's will be nanny, although when we talk to them about them, we'll make it clear by using their names too, so my mum will be nanny Sara if we say we're going to see her.
However, my mum last night questioned what my daughter will be calling my stepdad, and you know, we'd never even thought of it! My dad isn't around much, he's shown very little interest, so whilst we have to call him grandad, i don't really think of him as one. My OH's dad died last year, and i know he wants to tell hr all about him so he'll be known as grandad. My stepdad's kids have their own children and they always call my mum nanny even though they aren't related, so i just assumed that my stepdad was going to be our little girl's grandad too. My OH seems happy with this, and my mum says my stepdad will be honoured and that provided we were happy, he wouldn't be offended by it if we didn't. But i think its the right thing to do, aside from the fact that he practically brought me up, i think she'll get confused if her cousins call them nanny and grandad but she's supposed to say nanny and Pete.