It is never a bad idea to add some extra preventative under chassis protection. Depending on the area you live in and the road condition you may need more then what comes from MAZDA. Which is very little.
If you are a DIY and have a weekend to spend, buy a gallon of Silent Running SR CAR Thermal Insulation. Although it is not advertised as an under coating it will do very nicely in sealing things up and the main benefit of reducing road noise.