Big chain LFS have no clue what they're talking about from what I have seen.
Not really an LFS if it's a chain. I also work at a PetSmart, and let me tell you... people have almost no common sense when it comes to aquarium keeping. I'm talking both new hobbyists as well as employees (even 90% of managers have NO clue what they're talking about). If someone were to come in asking aquarium advice any day I'm not there, I know they will be getting bad advice
It's sad, but it's just a job to most people especially ones who are not hobbyists before employment. HENCE the most basic training is given to help try to bridge the gap between no knowledge whatsoever to a very VERY basic understanding. So this is why it is recommended to customers to do AT LEAST a 10% change once a week or a 25% change once a month. The reason for this is to remove nitrates, which as you should know are the last step in the nitrogen cycle; in tanks w/ just fish (like 99% of customers), this is the only way to remove them and keep them from reaching toxic levels. We just dont want people to have massive fish loss w/ new tanks, which unfortunately happens often when associates do not properly guide or inform new aquarium hobbyists. Planted tanks benefit from some nitrate removal via plants.
As has already been stated, we do larger water changes around here for multiple reasons, the biggest probably being:
With EI (estimative index) fertilizing, you essential over-dose your tank to make sure nutrients are available, then by doing your larger water change, you can remove the extra nutrients that aren't being used before algae is able to take off. Water changes do not affect bacteria if done properly, however if too large a change is made so as to greatly change water conditions and shock your fish, then you can have other issues.
It makes me cringe when I hear or see associates other than myself performing water tests for people and telling them their CRAP water is good and add more fish. I intervene when I can, but some people just don't care to learn. It's good advice to take what you hear from chain pet stores w/ a grain of salt, but if you're really lucky you'll get someone who actually does care and has some knowledge (like me