“How important is it that I don’t compress fiberglass batt insulation? It’s sometimes difficult to prevent during installation, especially around junction boxes, and I don’t want to negatively impact the R-value.”
Compressing fiberglass provides more R-value per inch, but reduces the overall R-value. That means, compressing fiberglass batts is ok if the cavity is completely filled. The North American Insulation Manufacturers Association (NAIMA) says: “When you compress fiber glass batt insulation, the R-value per inch goes up, but the overall R-value goes down because you have less inches or thickness of insulation.”
You can get a compression chart for R-value from Owens Corning:
When installing fiberglass around junction boxes, you will have to cut out the fiberglass where it goes around the box. Save the rectangular piece and put it in the space between the junction box and the exterior sheathing. This will require some compression, but since the cavity will be completely filled, it will be well insulated.