Sand deck or do not sand deck
There's rarely a reason not to sand other than the effort and minimal cost involved. It will usually result in a better final result to sand. 

You get varying advice because every project is different. You haven't shown any photos, nor have you described the condition or age of your wood or the products you intend to use. With all those factors, how can you expect a clear, black-and-white answer? It's really a matter of prerogative.