It depends on the shape of the gown - the silhouette you desire - but I will say two things:
1) if you have a hoop skirt, the gown MUST be tailored to fit over the hoop. It isn't this universal thing that you can just wear under any gown.
2) if you have a hoop skirt, you really should ALSO have a petticoat. Not the heavily-tiered tulle petticoat, but an underskirt-style petticoat. Otherwise, the hoop steel of the hoop skirt will show through your gown, especially if it's tailored tightly or made of certain fabrics. Nobody wants to see big giant rings, like panty-lines.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com