There probably is an official line on this. Myself, I usually find that different verb forms may collapse to the same end result. If you use DeepMeta to upload, you can easily experiment.
Also, the only foolproof way to know about a particular plural, I think, is to try it in the search box and see how the results match up against the singular. If they don't match then include the plural ... Though, again, you will often find that a plural collapses to the singular.