I use clear rulers a lot in my sewing, frequently in conjunction with Sharpie markers for pattern drafting, truing lines, tracing, etc. This causes buildup on the edge of the ruler (and sometimes elsewhere) that I don't want to accidentally transfer to fabric. Years ago, my friend Phyllis showed me that the best, non-damaging cleaner for clear rulers is good ol' rubbing alcohol. Just soak a paper towel in 70% alcohol, rub several times over the ruler, and voila - a sparkling clean edge.
It just takes a few seconds and it will ensure that your tools are clean and ready to use.