As a rule that's a good principle to adopt and particularly with chargers using only battery case temperature, a timer to detect end of charge or you don't know what's going on.
However, most chargers are now using delta voltage as well as delta temperature and time. Some also do a programmed discharge, before charging. So If you take a fully charged set of batts, let them cool down and repeat the charge cycle - you should be able to find out what's happening. You're more likely to find that fast chargers have the sophistocated end of charge circuitry which would sense a fully charged pack.
If the charger isn't discharging first, I wouldn't expect it to 'recharge' a fully charged set of batts in the same time and find they were very hot (i.e no delta V sensing).