I've heard the same for many types of of plants, not just ones in the aquaruims actually. I shall do the deed soon. Should you clip it as soon as you see it?
EDIT: I've also had an anubias that flowered, actually two, in a small 5 gallon. But these are really close to the surface already so they are good. I didn't clip them. This one sword flower is in a 20 gallon so didn't take that much to reach the surface either I don't think.