There are no adverse consequences to taking validation exams. They have multiple purposes. The primary one, of course, is to sort out who can be placed out of traditionally-plebe level courses such as Calc I and II, Inorganic Chemistry, and Physics. (Some departments also consider AP scores for economics and government.)
The other purpose of validation exams is to place students into the appropriate sections of the plebe courses above. So, take as many placement exams as your academic preparation suggests you might qualify for.
Here is one former plebe's and current professor's plug to do as well as you can on validation exams (you and everyone). Every year, there's a "should I do my best and get in a hard course, or slack off and get an easy course?" question. My personal opinion, supported by actual research, is that students who take courses that are repeats of courses they've already taken and passed, actually do slightly WORSE, as a population, on the repeated course than their first experience. This is probably because they think they know the material, and then do not do homework as frequently, study as frequently or as in-depth as they should. They have, as a group, a false sense of ease with the material and the course, and it ends up burning them.
The other arguments for validation are especially applicable to USNA. First, if you validate the course, it is on a rigorous, no-slack-afforded exam. USNA (and other rigorous institutions) do not validate people on whims, or on lucky guesses. If you validate a course, it is because USNA is confident that you know what you need to know in order to succeed in the follow-on course. Second, if you validate courses, you set yourself up for more electives or undergraduate research/independent study with one of the faculty in your upperclass years. I don't know whether the Master's program (Trident Scholar) is still active, but if it is, the Scholars almost always validated multiple courses as plebes back in the day.
USNA's engineering programs are accredited by ABET, meaning that you can complete all courses in four years of enrollment (assuming a student has the necessary and standard high school preparation of four years of the highest math possible, four years of science with labs, and four years of English.)
