ROTC follows the college's lead in things like this. If your future college doesn't care, then ROTC doesn't either.
This is a big topic at this time of year on college acceptance threads, for example, at College Confidential. Each school has its own guidelines. The most strict that I've heard of are UC Berkeley and UCLA. At the former, Senior year grades cannot dip below 3.0 UNweighted in either Senior year semester, with no D or F grades at all. UCLA is the same but the 3.0 UNweighted refers to the entire year, and not each semester individually (meaning a prospective student may have a 2.5 unweighted one semester as long as the other semester is at least 3.5 unweighted).