You want to keep the weight on a plane as low as realisticly possible. Balsarite and StixIt applied to the whole airframe is (IMHO) adding weight.

The ONLY time you actually need Balsarite or StixIt is with coverings that do not have any adhesive (or marginal adhesive) such as Coverite fabric, Mica film, Koverall, etc. It is HIGHLY suggested for using over oil/fuel soaked wood (after removing as much of the oil as you can ) and around areas than can not be "wrapped" (window openings, etc).

As for fuel proofing, while some people use it for that, I still prefer thinned epoxy for firewalls and clear Aerogloss dope for other areas (fuel tank compartment, etc). I KNOW those 2 items work, I am not sure about the Balsarite or StixIt for fuel proofing.