usually Muslim families prefer a Muslim maid(usually, not always) but that's because this maid is a nanny, too! which means she would be taking care of kids.. and that's why they want her to be Muslim.. So the kids won't be taught something that contradicts with their belief..
But if the family has no preference then that means that they woudn't bother her about her religion.. because the religion is listed on the maid's cv....
p.s. my aunt had a sirilankan maid who was a buddist.. and that wasn't a problem for them.. or for her..

another p.s. a friend of mine had a maid that worships fire.. and they had to change the gas tube(for the stove) every 2/3 days without knowing why.. until they knew that she wasn't cooking in the kitchen but praying ;p