How deep any substrate should be depends what kind of snake you're keeping, or even their age: some snakes love to burrow, & some babies start eating better when they feel "secure" (as by hiding-burrowing). When I raised rosy boas, they LOVED tunneling in several inches of Care-Fresh. Do make sure your UTH doesn't over-heat though...some substrates really "insulate" the cage floor (& don't allow the heat to rise very well where you need it, in the cage). All the UTH that I'm familiar with has instructions to allow some air flow...on a glass tank, that's easy to do. I use peel & stick weatherstripping (from hardware store) on the bottom edges of the tank, leaving several gaps for cross ventilation. If you happen to live in an earthquake area, as I used to, the weatherstripping will also help keep your tank from sliding off the stand. It goes without saying that all UTH should be regulated by thermostat or rheostat for safety too.
I often spot-clean a time or two, then do a full cage clean out when it's needed.
I have plastic 'critter cottages' for my snakes, where I put them while I clean their cage. I use white vinegar to clean water bowls (it disinfects & also dissolves hard water deposits); I use a weak solution of chlorhexidine in a spray bottle to wipe down (disinfect) the cages. (no need to rinse it, just wipe dry) Avoid using bleach, as it's very toxic & hard to remove the irritating smell (which would hurt your animals) if not rinsed so that NO smell remains.
Too bad you got such a negative vibe from the store: they have no room to talk if they aren't part of the solution to educate new owners. Sadly many pet stores are just minimum wage jobs with un-committed personnel...& many times you're better off without their (often incorrect!) advice anyway.