when do you prune a crab apple tree?

Only pruning instructions I've got say simply "remove any dead wood in winter, when the tree is dormant", so I'd guess pruning, as such, is limited to that.

This is the info from 'Landscaping America' site - seems you prune in late May:

Pruning Crabapple Trees
Crabapples require little pruning other than to keep it in shape or from interfering with other landscape specimens. Rapidly growing shoots from branches, called water sprouts, rapidly growing shoots from roots or the base of the tree, called suckers, dead, diseased, damaged, and crossing branches should all be removed. Sometimes on dense growing crabapples, it's necessary to prune the center of the plant to allow additional sunlight and air movement.

Pruning should be completed before early June. By mid-June to early July, flower buds for the next season are beginning to form in most crabapples. Pruning after July will reduce floral display and fruiting for the following year.

We prune in the winter, cutting back this years growth to 2 to 4 buds, always have an abundance of fruit surplus to requirements.