If you take a step, it's no longer standing tumbling. Go talk to my 9 year old Level 10 gymnasts, they do standing fulls with out a step-on balance beam.
But in cheerleading you only have running tumbling and standing tumbling. So step or not, if you're not running forward, it's still standing tumbling:)
Gymnasts always throw off the jump & tumbling sequence w/ those weird double arm swings (w/ out a step)..drives me crazy!