I either get the fish while they just arrived and are still in the original shipment bag, that way the new fish have less chance of being infected (fish didn't touch possibly contaminated LFS water) and they don't have to acclimate twice (LFS tank then your tank). You would need to know/talk with your LFS fish manager if they would do that for you. Mine know me personally and let me do that.
Or I wait at the very least, 2 days, preferably 3 or 4 days after they have arrived. I just give them that settling-in/acclimation period so they are less likely to die when you get them. When fish arrive and are added to a tank from just being shipped, they are highly stressed. A busy LFS environment is even more stressful for fish and they usually don't provide a comfortable setting for the fish (lack of hiding areas, people, seen as dangers to the fish, constantly looking at them and even scary nets dipping into the water chasing fish). If a fish will die from stress (which is common), it will usually happen within those first few days after arriving. Waiting the first few days, it is possible the fish will sell out, or die off, but if they are still there you would be getting fish that are more likely to survive. And it sort of gives you those few days to see if they end up developing any disease (if you happen to see the fish on arrival day and the day you pick them up. still quarantine new fish!).
If you can, it's best to fish out how long fish have been in shipment. If the LFS is good, they would have 1 or 2 days (max) shipment arrival time. If they used a cheaper shipment that took longer than that, the fish would stay in polluted water with little oxygen, and a bit of stress from sloshing around for quite some time, so they wouldn't be in the best condition. They may survive just fine, but the point is, the more stress, the less likely they will survive. Getting them if they arrived within 1-2 days shipment and taking them straight from the original shipment bags (before the fish are released out of the bags into LFS tanks) is almost like shipping to your door which is the best option.