Should I be worried about the waste from "sugar rockets" and other potassium nitrate fireworks?
Potassium nitrate is transformed into potassium nitrite only when heated alone. And it is not really toxic, as it is added in ham to prevent its too early oxidation in air.  When heated with charcoal, sulfur or any other oxydable substance, potassium nitrate is transformed into $\ce{K2CO3}$ or $\ce{K2S}$. For example, black powder is a mixture $\ce{KNO3 + C + S}$ that reacts violently according to ; $$\ce{2KNO3 + 3 C + S -&gt; K2S + 3 CO2 + N2}$$ And if the explosion is made in contact with air, the hot $\ce{K2S}$ is quickly oxidized into sulfate according to $\ce{K2S + 2 O2 -&gt; K2SO4}$
Sugar rockets are made of ordinary sugar $\ce{C_{12}H_{22}O_{11}}$ and $\ce{KNO3}$. This mixture reacts according to the two following equations : $$\ce{C_{12}H_{22}O_{11} -&gt; 12 C + 11 H2O}$$  $$\ce{4 KNO3 + 5 C -&gt; 2 K2CO3 + 2 N2 + 3 CO2}$$
So the ashes of any combustion reaction are mostly made of $\ce{K2CO3}$ which was the main constituant of the wood ashes. These ashes was used in the Middle Age for washing clothes by our grand-grand-mothers. They are not considered as toxic