Is it necessary that I charge my batteries to 100% each time I run the charger or is 70% or 80% good enough? I know that if I stop at 70% or 80%, I will have to charge sooner. I am concerned that if I start not charging them completely, over time I will damage my batteries.
While cruising it is totally fine to stop charging before 100%. Furthermore don't bother to charge past 80-85% unless you have AC shorepower or are underway and going somewhere. Get to shorepower once a week or two to get your batteries to 100%.