Depends on the pack and depends on the way its placed. I have an intense dislike of the standard issue EMS/FA heat packs. They heat up too high, burn out too fast out date quickly and are prone to puncture and accidental activation. I much prefer the slower heating foot and hand warmers sold in retail. They hold their heat longer and are a much more gentle heat. In SAR I carry ziplock bags of ordinary deicer. Added to water from your water bottle or any available water source (not necessarily drinkable water) they create a nice little heat pack placed in a plastic water bottle, nalgene bottle or other water carrying device. They are light and durable and don't get activated until you add the water.
If you are using the regular EMS hot packs, you need to carry some sort of wrap so the heat isn't in direct contact with the patient's skin. We carry rolled stockinette cut to fit. We have also used towels or face cloths to use as a barrier. Aside from that issue, if you keep the heat packs close to the core instead of at the extremeties, you should be fine.