The reason I didn't include Vicki in the poll was that it would be a foregone conclusion in the minds of many that she would be the first choice, especially since she is most readily identified with the role of governess, and served the longest as governess ( about 2 1/2 years before her mysterious disappearance into the past with Jeff Clark/Peter Bradford).
I think I included just Maggie and Daphne in this poll since apart from Vicki they are the only two governesses who are both seen in the 20th. century. And its true Daphne was in a competition of sorts as a matter of course with Maggie when she transcended time to live in 1970.
I admit it does seem vague to have said "in any time period". I suppose I could have said "in the same time period" instead, since that was the case with Maggie and Daphne. I feel these are the reasons I narrowed my choices down to these two, and did not include Vicki or Rachel as choices.
Though Daphne was not appointed a governess in 1970, she would possess the knowledge and experience of her having been a governess in 1840. Another thing about DS characters who transcend time to live in the present: They seem to fit in as naturally in the present as they did in their own times. Thus being out of her own time would not seem to be a disadvantage for Daphne.
I hope this post helps clarify things a bit as you consider the choices in this poll.