It is must my dear. This will make your trekking easy and if anything goes wrong with you then the agency will be at the back of you for your help. Without them your trekking will be a mess. Trust me! Moreover, they will assist you with guide, porter, equipment, flight tickets, bus tickets, gears and many more. Why not go with agency! After all, you are here for enjoyment not to regret. And, do not forget that agency will help you if you are clutched by altitude sickness. They will evacuate you from the mountain by sending helicopter.