Hibiscus rosa-sinensis, also known as Tropical Hibiscus, needs to be protected from cold temperatures as it can be damaged by frost and freezing temperatures. Typically, you should bring it into a protected garage or inside once overnight temperatures are forecasted to drop below 50°F as the trunk will die and the roots may not survive (you can try providing plenty of organic mulch) in very mild winters.
Hibiscus moscheutos, also known as Hardy Hibiscus, Rose Mallow, Swamp Rose-mallow, Crimsoneyed Rosemallow or Eastern Rosemallow can be pruned all the way down once it goes dormant and the trunk(s) dries out.
Hibiscus Coccineus, also known as scarlet rosemallow, Texas Star, Brilliant Hibiscus or Scarlet Hibiscus can also be pruned all the way down once it goes dormant and the trunk dries out.
Hibiscus syriacus, also known as Althea or Rose of Sharon, can be left alone outside or you can prune it all the way down once dormant.