Plums and other 'stone' fruit, such as peaches and apricots should never be pruned in the winter, but wait until the weather warms up in late spring. This is to help prevent them from getting a disease called silver leaf. Pears, apples and other 'top fruit' should be pruned now, before they start into growth. If they are pruned when the weather is too warm it prompts them into making loads of soft whippy growth, which is undesirable.
I plan on pruning my two pear trees some time during this coming week.