You must be careful of transactions that have not been confirmed or that receive the fewest number of confirmations, and even that many currencies that require confirmation or two are not sufficient because the difference between each block and the second is much less than 5 minutes.
for bitcoin 1 confirmation is enough for small transaction is good and its is good  to wait for 6 confirmation
other altcoins is good to wait for more than 10 confirmation and i remember that Bicoin satoshi  need 1700 confirmation