Printing can be quite wonky sometimes. I'm not sure if Splunk would pickup a print.css but that is generally the best way to ensure you get sensible print results.
If not you could create a duplicate app with a custom application.css that has settings to enhance the "printability" of a dashboard.
An other alternative is to just tweak your dashboards a bit more to try and get better layouts and views on print. Out of interest what result are you getting compared to what you expected?
