That is why I vacuum the heavy dust out first... Sure a vac can cause static shock, but that's why you use metal hoses/wands that are grounded to the case.
I have on a very strange occasion, but only due to dust, dry air, and me using plastic wands. Was enough to burn out a stick of RAM, since then, metal wand only, and it's grounded to the case.
In 2006 I custom painted a case black for an ex (gf at the time) of mine then slapped a PC together so she could game on it. Earlier this year I actually got it and most of the components back. Yea, the black is now a light shade of gray after cleaning the case out. That is vacing the dust and then using an alcohol/water mix spray with a cloth to clean it. Some how the dust was so bad it penetrated the clear coat and stained the paint job. Turrable. Oh yea, reason she stopped using it. The dust was so bad it literally stopped the CPU fan from spinning so the CPU died from heat.