I show them whenever they fit the needs of the buyer. What I think is even a bigger reason to show them is that alot of buyers in my area will call the fsbo signs either before after or while working with an agent. I hear it alot from agents in my office, they get that phone call " I'm sorry but we found a property that was being sold by the owner and he said we would save money buying directly through him".
I don't avoid them on purpose. However, if the FSBO listing is incorrectly entered into the MLS or not in the MLS it might be missed. For any listing to make it into the tour it has to be reasonably priced and generally meet the prospective buyers search criteria.
On the contrary, I actively seek out FSBOs, cancelled listings, and expired listings, in addition to the active listings on the MLS, to be sure we are seeing everything that could be the potential "it" for my clients.
As long as I'm aware of the FSBO, if it fits my clients needs, I show it. If the commission structure is unknown, I will contact the FSBO in advance to determine how the co-op fee will be paid, and make arrangements, in writing, prior to showing it.
I would be doing my buyer-clients a terrible disservice, by not showing the a home that fits their needs.