It is a good idea to charge deep cycle lead-acid batteries to full charge. If you frequently charge them to less than full, eventually excessive lead sulfate will build up on the plates and the battery capacity and performance will be reduced.
There are chargers that will "de-sulfanate" a poorly performing battery, but the process of changing the lead sulfate back in to lead and sulfuric acid also allows some of the lead to fall
to the bottom of the battery (this happens to a lesser extent with normal charging too). Your battery will again perform, but at the cost of reduced capacity. The best life span is achieved by regularly fully charging.
Another important thing about batteries is to make sure they are fully charged before storing them, "top them off" by charging them every few months while in storage, and check the water levels of the fully charged battery. A good solar
powered battery maintainer is very handy for this -- not just a solar panel
hooked the battery. Preferably one that does a temperature compensated float charge.
In any case, battery chemistry "marches on" and batteries will eventually wear out and need to be replaced, it's just a matter how often based on how you care for them. If you understand that you might be replacing them a little more frequently, use them however you want.