It is a good idea for brokers to look at each website with an IDX feed. At the same time there are hundreds of IDX feeds and checking each one is not feasible for most brokers.
Zillow is not favored by some brokers because its value estimates can cause problems with buyers and sellers. For this reason I do not visit Zillow.
Most brokers deem the MLS the most authoritative source of information, even though it is not perfect. Brokers nationwide invest heavily in MLS data to make it the most comprehensive and accurate source available.