This threshold is based upon research conducted by the Building Research Council at the University of Illinois. Field tests of residential water systems subjected to winter temperatures demonstrated that, for un-insulated pipes installed in an unconditioned attic, the onset of freezing occurred when the outside temperature fell to 20°F or below.
Pipes exposed to cold air (especially flowing air, as on a windy day) because of cracks in an outside wall or lack of insulation are vulnerable to freezing at temperatures above the threshold.
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So any visible pipes should be wrapped w/ newspaper and taped up to prevent transfer of heat. Set a trickle at 25°F to be safe. Trickle is not a steady stream. Trickle is like right before it makes a small stream like 4-5 drips per second. A trickly will most likely be cheap than electrically heating the shop.