Anthony,
That really depends on the airplane and the application. Spoilers serve a number of functions, including acting as speed brakes, devices to dump lift and put weight on wheels for landing, and serving to aid the ailerons in lateral or roll control in flight.
The way the spoilers are used varies with the airplane. Some airplanes use a set amount in flight, others use variable amounts in a speed brake capacity...some use them for roll control, others don't.
In our airplane, the speed brakes are auto spoilers on takeoff, and they're not armed. If we reject the takeoff, the spoilers will autodeploy when the second and fourth reverse levers are moved past the interlocks and into reverse. On landing, however, we arm the autospoilers and they deploy when the first and third thrust levers reach idle, and the weight is on the wheels enough to untilt them.
In flight, we use spoilers during a descent when necessary. It's generally preferred to not use them unless necessary; they make noise and vibration, and it's better to plan the descent and arrival without them. When given a late descent or a reduction in speed, however, they're a legitimate and useful tool for slowing down or going down, or both.