The life cycle goes for a little while and it is always best to wait for a week min. after the last spot is seen but I would wait about 2 weeks to help the fish get back to normal. And be sure the little Ich bugs are really gone.
Bringing in new fish with new possible illnesses to pile on top of the already weakened immune system is a negative.
As mentioned above using a QT practice can be especially important in expensive fish, sensitive types of fish, scaleless fish and inverts in the tank which do not survive many types of treatments, and just simply the security of an established tank.
QT practices vary in many peoples definition. A week would help you see glaring issues, a couple weeks min, will help you make sure there aren't any big things been brewing and the fish should be looking pretty good. But some like longer periods.
It can also depend upon if you have an imported fish, if they spent awhile in shipping prior to you getting them, if you treat them when they arrive, deworm them, wait to make sure any parasite issue would be cured.
Frequent the lfs
where they came from and can get accurate information from the manager about health of the fish from that particular tank coming down with something...