Ok, I cleaned my two today and it DOES SMELL! It smells a lot. I had to wrap tissues around my face (ha!). When you open up the anal sac what do you see? If you see brown/black then it needs a desperate clean. If you have a regular routine of cleaning then you will prevent impaction.
Taken from guinea lynx:
In older BOARS, (rarely younger and rarely in females because the sac is much smaller) the muscles of the anus stretch/weaken and the piggie is no longer able to properly expel the fecal pellets from the anus. Both caecal and fecal pellets become trapped or "impacted" in the perineal sac. If left unchecked, your boar will begin to lose physical condition because he is no longer able to eat the caecal pellets and the blockage interferes with the normal digestive process.
The perineal sac is a collecting place for all sorts of debris (hair, cage shavings, hay) and should be cleaned out on a regular basis