I usually refer to
this table from 2008, which shows the most commonly affected chromosomes and whether they indicate a relative better or worse prognosis.
Chromosome 18 doesn't appear in the chart, so it must be a less common abnormality, as you've observed.
I spotted a 2006 report of a
study of complex chromosomes in MDS and AML patients in which they found that 4 of 17 patients had abnormalities in chromosome 18, so it's not unheard of. However, they were studying only patients who also had chromosome 5 abnormalities, and chromosome 5 is much more predictive.
My wife had a similar situation, with uncommon abnormalities for which we couldn't find research results. Her doctor said that having no abnormalities is better than having any abnormality, that having an non-studied one is better than having one with a known negative prognosis, and having abnormalities that are stable is better than finding different abnormalities from one bone marrow biopsy to the next.