Is there a way to hide hotbars when not in combat?

[Risvertasashi]

Sort of. If you're looking for it to be completely automatic, I don't think there is. But you can make macros to hide (well, toggle really) pretty much any hud element, including hotbars.

For example

/hotbar display 1

putting this in a macro will toggle displaying hotbar 1 on or off. Add the rest of the hotbars you want, and any other hud elements you might not want on all the time.