I usually check scent glands when I trim nails, which I usually do every 6-8 weeks. (I know the recommendation is once a month, but my bunnies' nails don't seem to grow that fast?) It's just part of my regular check over; nails, scent glands, brushing, look at the nose and eyes, and a general pat down to feel for anything irregular.
Tobi is always squeaky clean.
Nick sometimes has just a bit of buildup; I think it's because his fur is so long and thick, he has some issues with cleaning.
Amelia......is usually pretty gross in that area. I'm not sure if she just doesn't clean herself well, or if she has problems due to her roundness. She sometimes gets a between-inspection cleaning, if I can put my nose next to her and smell the funk, it's time. :rollseyes