Michael,

Here is a link to CSU factsheet on how to prune shrubs based on when the shrub flowers. 

https://extension.colostate.edu/resource/pruning-flowering-shrubs/

For winter pruning, you can always remove dead or damaged wood, or any branches that are crossing over each other on the inside of the plant.    

Next year's flowers with appear on this year's new growth. If you prune the shrubs this winter, you will cut off the buds for spring flowers.  Ninebarks are generally pruned after they bloom so the shrub can develop new branches and set flowers for the following year. If you want to do heavy pruning (like rejuventation pruning),
realize you will not having flowering in the spring. Read the article
linked above to understand a bit more about this type of pruning.  You may want to consider thinning in the spring.   Ninebarks can benefit from periodic thinning.  Again refer to linked article.

Jefferson County Colorado Master Gardener Brenda