Yes, it's always safe to assume - but your success rate will go way up if you do a little scouting. Remember, you can't call predators if they're not there! Where they are at night, they may not be during the day, and vice-versa.
Predator scouting is easy - and fun. When you're out hiking around, check for tracks and sign in creek beds, game trails, and fencelines. These are areas predators will most often be using. At night, try to locate where they are howling from, or use a siren or your own howler call to locate them - but watch out! They may come in for a closer look, and if it's legal to hunt at night in your area, do so - that's some of the best predator hunting you will do.
Ladder stands or tree stands work well for predators, just place the call on the ground of course! You will come upon predators from time to time when you're just out walking, but most of the time you will not see them. Every now and then, though . . . . last week I was creeping up to a stand location just as the sun was about to set, and caught a coyote laying down in the field about 200 yards ahead. BAM! Dead coyote, didn't even have to call. So it happens, but not often.
Predators will come from a long ways off if they hear your call, so give your stands plenty of time, depending a lot on location and time of year. I have stood up from a stand a few times and seen a coyote or such making haste the other direction - sometimes even after 45 mins of calling. You just never know when they'll come in - which makes calling so exciting!