We never had issues staining macrophages with Hoechst 33342 but we used to get it from Sigma and dilute it ourselves. Maybe you could try diluting the reagent (reduce the number of drops per mL) or reducing the incubation time.
As Julie, I have used Hoechst 33342 also in various cell types for life cell imaging.
I would recommend to use the lowest possible concentration in combination with a fast and sensitve imaging system. Due to the fact the you are using short wave length light (near to UV) to visualize the Hoechst you are of cause generating ROS within the cells.