If the vehicle is an automatic and the transmission transmission is left in park then yes it will be ok. However it is advised that the handbrake is used when the vehicle is parked.
If its a manual vehicle, then you should apply the handbrake even if the car is left in gear. Not just when you're around hilly areas either, as strong winds can cause a vehicle to start moving as can passing trucks etc. Also say the vehicle is hit by another vehicle whilst parked.. if there is no handbrake applied your vehicle would then be able to go rolling away causing perhaps another accident or injury to someone else.
