Something I've posted before: local stores are not charities where we should be making bad purchases just to feel good about it. They are businesses that should get our business because they offer goods and services at good prices. On the other hand, no, it's not ethical to take advantage of their services without paying for them, which seems to me is what "showrooming" is.
With musical instruments and the internet, it's good sense - individual instruments, even of the same model, can vary a lot from one to another. If I like an instrument I try out in a store, I want to buy THAT instrument, not another one that's available online.
Likewise, used instruments are cheaper than new ones for a reason. Part of it is the wear and tear and the risk that you're buying someone else's headache. Related to that, buying a new instrument at the store gets the store warranty and service behind it.
Bottom line being, even at a higher price, buying from the store may still be the best decision. But it's not an absolute black-and-white decision.
I do try to make a practice, at stores I visit regularly, to buy the lesser stuff from them on a regular basis - strings and picks and cables and straps, taking basses in for setups and things like that. So that at least they're getting some revenue for the service of making instruments available.