I'd still quarantine, regardless.
The main thing with quarantine is to keep the fish in a STABLE environment for a month. Even if the fish has been at the LFS
for a month, new fish (and possible diseases) are coming in every week. So even if the fish had been there for 4 months, the thing could've picked up a parasite from the batch of fish that were added the day before. Even if the fish are in separate tanks, the water source is normally the same between all the tanks.
Same thing goes with your own home quarantine... if you have a fish in your quarantine tank for 2 weeks and then add another fish, you've just reset your "quarantine clock" back to zero for that first fish.
And then again, many folks don't quarantine at all... so it's really a personal risk/choice.