It depends on the chemistry of the battery.

----- Lead-Acid batteries greatly prefer to be charged ASAP and should not be discharged below 50% before being recharged for best battery performance and life.
----- NiCd and NiMH chemistry batteries should always be discharged all the way or nearly all the way before being recharged for best battery performance and life. These are the batteries that really have a memory effect, especially the older NiCd chemistry.
----- All Lithium (Li) chemistry batteries should not be discharged below 20% before being recharged for best battery performance and life.
----- Most Lithium (Li) chemistry batteries should be charged to about 75% of their capacity and not charged up all the way if you intend to put them in long term storage without use for quite a while for them to hold their performance and life best over the storage period (unusual compared to other battery chemistry which usually store best starting with 100% full charge). This is important to know if for example you only your e-bike during the summer months and store it during the 3+ month winter season, if you use any of the more common lithium chemistry batteries the best way to store over the winter is to only recharge the battery to about 75% capacity and pull the charger off short of full charge and then store the battery with nothing hooked up to it to drain power, then give it a good charge to 100% before using it when you pull it back out of storage.
----- Most Lithium (Li) chemistry batteries have a minimal memory effect compared to NiCd or NiMH chemistry. I wouldn't recommend continuously only discharging them to say 50% and never taking them down all the way to 20%-25% every once in a while but after a short ride its not a problem to throw on the charger and top them off on charge, just be sure to occasionally take a long ride and pull them all the way down to the 20%-25%.
----- There are differences between different lithium battery chemistry types For example Li-Ion batteries have more of a memory effect then LiFePO4 and should be taken all the way down to 20-25% more often.

On all those % I gave I am referring to how much charge is left in the battery. When I say run a Lithium battery down to 20%-25% every once in a while to prevent memory effect, but don't run it down below that if at all possible, that would be like running a car down to a 1/4 tank of gas left and not going below that before refilling if at all possible. Storing for long periods (months without use) at 75% would be like storing a car with 3/4 a tank left rather then filling it all the way up full.