I went to high school in Texas and we had a homecoming king and queen, but we did not pick them again for prom. The homecoming king and queen were seniors, and they were voted on by a process of nomination then election, with one person from the major cliques (band, sports, nerds, richy-rich kids and non-white kids (disclaimer--this is how it was seen broadly, not by me specifically; many nominees crossed boundaries and thus were exceptionally popular)) generally making it onto the court, and the jocks winning in the end. Some years a special ed kid or very popular nerd would have the edge, though.
Aside from the ceremony at halftime of the homecoming football game (all the nominees came out wearing their homecoming dresses or suits, and the winner was announced, with the girl given a big bouquet and maybe a tiara or sash), I don't recall it meaning a darn thing. We didn't have a parade or a special dance for the winners or anything. The nominees *might* have had to do a skit at a pep rally, but that's about all I recall.
We also had the 'superlatives' but I don't remember voting on them really mattering -- the yearbook staff had the final say, so their friends were generally picked as that grade's "most popular," "best dressed," "best couple," "most likely to succeed," etc.