I am no expert on this, but here's what little I know. A Disney Cruise will be more expensive in the summer and over major holidays, since kids and others have time off school/work. Off-season will be best for you. We haven't been on a cruise but have our first scheduled for December. The week before Christmas was much less expensive than the week of or after Christmas. If you're considering a cruise, I'd recommend buying the Passporter book on Disney Cruises.
I think this is a rather open ended question, since "the best" time to travel for some may not be the best for others. I would have to ask what is the "most important" aspect for you? If its price then yes the rate does vary with the time of year and tend to be more expensive around the Holiday and summer but not always. If it's weather then early fall may be cheaper but it's also hurricane season and weather could impact your ability to enter some ports. If you want a long relaxing cruise then the 15 day repositioning cruises may be for you.
There are so many options when sailing with Disney and while none of them are "bad" options I would recommend working with a travel agent who knows and understands the DCL and can help you make choices that are THE best for you. Any of us at Magical travel would be happy to assist you. Our services are free, we can help you plan a wonderful Disney Cruise, and we can often help you save some money on the trip as well. So please let us help you.