Retailers are the ones who heavily promote that to get you to buy more memory.
Most users aren't going to notice the difference between 2-4 DIMMs where capacity isn't a factor, the difference between 3 and 4 is tiny for content creation workstation performance.
Buy DIMMs from the same manufacturer with the same specifications you'll be fine. Filling all 4 channels shouldn't be something to try attain, just go for the capacity you want.