Of course not. Some agents offer more. Some offer less. Even at the same agency, different agent offer different amount of onboard credits. If you are just look for the best price. There is a website called cruisecompete where you just fill out your request and agents will send you quotes showing what discount or onboard credits they can offer. And you can choose to contact the one suits you the best. From my experience, there are agents offer great discounts or obc about 10% of total cruise cost with direct bookings. But these agents don't know
Disney Cruise well as I do. So I have to do a lot of the research myself which I actually really enjoy. So you may choose between agents who offer great discount but can't give you great advice on disney cruising and agents offer less obc but know Disney very well and give you tips of every step of your trip. I will choose the discount since I enjoy planning the trip myself.