The weight of the car will be sufficient to keep the spring from decompressing fully and pulling out the bottom of the strut frame in the car.
You must be careful not to jack the car up without the bolts secured tightly though, this could produce a dangerous situation. Also if you are ever working on the 'southern' half of the spring, where it interacts with the wheel arm, you MUST use a compression tool as it is under pressure and can shoot off when the bolts holding the assembly together are loosen.
You must always be extremely careful when working with any system that is under pressure, whether it is from a spring, motorized or from hydraulics. Check with you vehicle's manual before performing work so that you are familiar with your specific car's systems.