Do not confuse ambience with room-tone. Room-tone is the sound of the set when no one is speaking. Even when shooting out-doors it's called room tone. If there is a distinctive ambient sound make copious recordings to blend in with the room-tone. For example, when I worked on "Trailerpark" the trailer park was near a lake that had frogs chirping all night from July through September. Fortunately the production sound mixer recorded a solid 20 minutes worth for me.
Ambient background sound is constructed of multiple layers of sound so it can be controlled. With most sound design there must be visual cues so there is a reference. When it comes to ambient BGs however, you can reverse this; the sounds themselves can create the illusion you want. Grasshoppers, locusts and tweeting birds easily convey an afternoon in late summer. Slow traffic, honking horns, a few voices and maybe a distant siren suggest a city. Whistling wind can suggest cold, but add thunder and rain and it can suggest a hurricane, which only takes place during warm months.
Going back to "Trailerpark" I used the frog sounds and mixed in some crickets as well. The location was actually fairly close to a busy highway, but I stripped all the traffic noise out and by doing so kept the location more remote as the director wanted the trailer park to be an isolated semi-enclosed world. I leaned very heavily on water fowl sounds (ducks and geese) for the autumn, and lots of birds for spring. The summer was heavy on grasshoppers and locusts (but no birds) during the day and, as I mentioned, frogs and crickets for summer evenings. The winter had lots of cold wind and cracking ice (the Foleyed footsteps were crunchy snow) - we wanted it to be very cold all the time. It was not unusual for me to have 20 layers for the ambient BGs.
For horror it's going to depend upon you setting. You may want a very "empty" ambience to isolate your characters. The "emptiness" magnifies small sounds - creaking floors, skittering rodents and the like. If it's out doors you may want a normal ambient BG for the situation and then eliminate it all as the danger approaches.
This is the essence of sound design, creating a complete sonic world that communicates to the character and the audience.