Flower must be fully open but 100% fresh - maybe one tiny row of petals right in the middle still closed. Once those little yellow "flowers" in the middle start to appear, the zinnia is past its prime - in terms of showing. Hope that answers your question. With most flowers - the standard is the same - very very fresh and no insect or other damage.

Thanks Shapiro. It seems to me that some zinnia flowers have those little yellow florets right from the start and that was confusing me. I was waiting for them to disappear rather than for them to appear.I actually like those little collars!