I tried it on my way in today, though, and it was kind of fun trying to figure out how NOT to stand up.
If you encounter any other riders on your commute, you'll also have fun realizing that you've already made it across the intersection after the light turned green, while they're left in your dust, still barely getting started.
Or, conversely, you'll finally stop being left in the dust by those who know how to use their gears. :-)
The first situation is more likely though, because in my experience, those who don't know how to use their gears outnumber those who do. So you certainly aren't the last one to learn. It's not something that's taught very often.
30 years ago, such active shifting wasn't too practical, since shifting generally required you to take your hand off the handlebars, and was a bit more finicky. But these days, most bikes have shifters located right where your hands are, making it super-easy.
I downshift at every intersection, every time I slow a bit to see if a backing car sees me, at every tiny incline, and even when the wind blows a bit stronger.
Watch out though, when I taught my girlfriend how to ride a bike (from scratch), I immediately incorporated active shifting into her training. Now there are times when she complains that she needs "more gears" because one gear is too high and the next one is too low...and she has a 24-speed!
While we're on the topic of things-bikers-aren't-taught, it's probably worth checking your cadence too. There's a good chance your cadence is relatively low, and you could be a more efficient rider if you increased your pedaling revolutions-per-minute. 90rpm is a rough "ideal" target for many people, and at first that might feel funny to you to spin that fast and "lightly" if you're used to standing and stomping to get the bike going. The whole point of gears is to allow you to keep a constant, ideal cadence, no matter your speed, hills, wind, etc.
ETA: hopefully the shop workers didn't actually say that not-shifting is "a terrible thing to do to your bike", because it's not. Yes, it will concentrate the wear on one gear, but people with single-speed bikes concentrate the wear on one gear too. So at most, it's a waste to *have* multiple gears and not use them, and will increase your maintenance costs somewhat (since replacing your one-of-seven worn gears will cost more than replacing the one-of-one gears on a single-speed), but it's not going to make your bike explode or anything. The main reason to use your gears is because of the benefits they have to *you*, not your bike.
