However if the minum valley is above the droput then the ripple won't be too bad.
The amount of dropout voltage is TYPICAL, it is not guaranteed. Some are worse.
I agree; in my opinion datasheets should always provide the worst case as well as the typical. Another thing to be aware off is the output tollerence, the LM317 has a tollerence of 5% so don't be suprised if the output is 5.25V when the resistors are calculated for 5V; the 0.25V should be added onto the dropout voltage requirement, hence my minimum 250mV statement.