If they have dead, damaged or diseased wood, cut it out when it occurs to prevent further problems. I prune my fruit trees, including crabs, in late winter, just before they all wake up. That’s the traditional time, and when you will see orchards and ornamental fruit crops being renovated typically. Start with suckers and water sprouts and so forth: wood that can never amount to anything good. My basic pruning guide is here: http://awaytogarden.com/pruning-pared-way-down As member Cherzeca says, when you have the time…meaning when it will get done…is when to do it, as with any pruning task, so if late winter doesn’t sound ideal, the trees are somewhat flexible.