That's the first I've heard that it doesn't show up on a neg. It would seem that it would be best to keep the light path clear, because whatever is in it is going to be, you know, enlarged on the print.
In most cases those surfaces are a long way away from the negative, and the depth of field/depth of focus at the negative is quite shallow. Nothing on the surface of the condensers will be at all close to being enough in focus as to be visible in your results.
If you have enough dust and crud it might cut down on light levels, and if you really have a lot of dust and crud you might add some additional diffraction, but if things were that bad the condensers would look terrible.
It is a bit like dust on the front of a camera lens. Except in the case of ultra wide angle lenses and small formats, the surface of the front of the lens is so far way from the plane of focus as to make it impossible to image the dust.
Dust will add diffusion due to flare, but it seems that you actually are looking for a bit of that.
The one condenser surface to pay attention to is the one right above the negative. You won't see the dust while it is on the condenser, but it could fall to the negative, where you will see it.
If you want to demonstrate for yourself how invisible something on the condenser is, put something temporarily there, take out the negative, and see how much you have to rack out the focusing of the enlarger in order to have an image appear of your addition. Depending on the condensers, and your choice of location, you may find it impossible to see any evidence of that addition, no matter how you adjust the focus.