Generally you would want the curtains to come to just above the floor, so as not to obscure the vents when the curtains are closed, consider giving them an extra inch of clearance.

I have the same question - we just added on a new family room and the vents are right below where the curtain stack for long panels would be. I've heart about vent flow covers, but not sure if they are a good idea.

letting the drapes puddle helps, i did that with my silk drapes, i wanted the lush look even tho its out of fashion. i can push them back to keep the floor vent clear.

try fishing weights in the hem.

i'd skip panels completely if the correct length (kissing the floor) didnt work.