That can be a hot debate between CP's and NCP's......
All I can tell you from our experience (CP and NCP) is that it's different between DS and
SS. In DS's case, he was born with a ready-made step-mom (long story). When DS started to talk, he couldn't say 'step-mama'...it came out 'Ep-mama'...and it stuck! He's 22 now and still calls her that, LOL! He's pretty close to her and I'm glad that he still feels comfortable calling her that....she even signs her cards that way!

DS's dad died in 2002. And DS has always called
DH by his first name.
In SS's case, I came into his life when he was 3 and he calls me by my first name. That's what his Daddy called me, so that's what he called me. We had an issue when SS was 5 with last names. Apparently when
BM got remarried, SS thought that his last name changed, too....and BM never corrected him. That came to a head when we saw that she'd labeled SS's toy bag he brought to our house to include her husband's last name. DH put an immediate stop to that. To be honest with you, I'm glad we just automatically used my first name...BM would have gone ballistic if SS called me mom, too.
About 7 years ago, we were camping with both boys and DS asked about doing something and SS answered 'If Mom will let us'. I whipped around and looked at SS....he has a little grin on his face, he didn't do it by accident. But he also knew better even then (he was about 9-10 at the time) that his mom would hate it. It's happened a few times since then, but I realized he only did it when DS was around, because that's what DS calls me...I think it was because it made SS feel more a part of the family.
IMO, it can be such a volatile issue that a suggestion I would make is to ask the bio-parent how they feel about it, whether it be a first name or even a pet name. But if that's not an option, I would stick to first names....very young children might find it easier to have a pet name, though. But I feel that, out of respect of the bio-parents, step-parents should never be called Mom or Dad or insist a child do that. Once the child becomes an adult, what they call a step-prent is up to them, even if it's 'Hey you!