The water ingress sticker is to keep any liquids from electrically connecting the covered electrodes. You definitely want to keep them covered. I do not believe water will infiltrate the entire battery since the cell is hermetically sealed within the plastic rectangular casing. However, the protective circuit and other circuits of the battery at the location of the electrodes could also be compromised if you don't have the ingress sticker in place and some water gets in.
Dropping the battery from "reasonable" heights is probably ok especially if dropped on carpet. However, this doesn't guarantee anything. Usually, the battery element is a laminate (anode, separator w/ electrolyte, cathode sandwitch). So, if you dent the casing, the individual layers forming the laminate might short together through the separator and the battery will be compromised/unsafe. Certainly, if you crack/dent the hard plastic, I would not use the battery.
