Can I brake without de-clutching?

Each gear has a range of speed that you can drive in. Keep in mind that gear ranges overlap each other.

As long as you drive within this allowable speed range any modulations in speed are usually moderate enough not require a gear change, and therefore no need to depress the clutch.

If your speed drops below the allowable range of the gear you’ll need to depress the clutch pedal to either downshift, or to stop.    

Anytime that you depress the clutch unnecessarily you coast. Coasting reduces your ability to control situations because you lose your ability to control your speed with the throttle. You also contribute to the wear factor thereby shortening the useful life of the clutch.

Keep it simple and depress the clutch only when you have to.