Its not a big deal. If you find nav interesting to learn then go for it.
The only thing to watch with the nav exam is that although its multiple choice like the others, you do have to work things out on the chart and with the whiz wheel/triangle of velocities to a dgree of precision that is quite challenging (probably more precise than you need in the real world, in my view). A further twist is that some questions rely on you having got the answer right to an earlier question - so if things don't make sense then you'll need to go back and check.
As with the other exams there's plenty of time though.
Best of luck