Thank you for contacting us regarding your question.  Ash trees are not fruiting trees nor do they "bleed" like other varieties, but there is the other issue.  Most should be pruned when dormant (winter), but the other time for ash trees is fall.  "In the past, borer was active in May but it is possible for it to be earlier. To not get into the time of active infestation of living branches, that is for shaping, fall seems safer. Dead or clearly diseased branches can be taken down anytime. All pruned wood should be treated as infected." Minnesota extension.

Here are two good resources about appropriate pruning.
Purdue University: 
https://www.extension.purdue.edu/extmedia/ho/ho-4-w.pdf
University of Minnesota: https://extension.umn.edu/planting-and-growing-guides/pruning-trees-and-shrubs