Most sources say that conventional deep cycle batteries should be discharged to at least a 10% DoD (depth of discharge), but not more than 50% DoD before recharging for maximum life. AGM batteries should also be slightly discharged before charging, so float or solar chargers will not be the best choice for maximum battery life with these types of batteries. Better charge controllers for solar systems are designed to prevent overcharging, and also to prevent charging until the batteries have been discharged the proper amount. The convenience of not regularly bringing them in for charging may be outweighed by considerably shorter life unless you buy a very good solar charging system that has a good charge controller. Float or maintenance mode is great for maximizing lifespan of regular lead acid car batteries, but not so good for conventional deep cycle or AGM types. See section 7 of this link. It is one of the better ones on the internet that is still easy to understand:
http://marine-electronics.net/techarticle/battery_faq/b_faq.htm
