Back heat is great if the room is heated already to around 75 deg or more. If the room gets cooler than that, better go with belly heat.

If you run a continuous piece threaded through the shelves, you only deal with a single plug. Certainly the cleanest-looking solution.

If you run individual pieces each with its own plug, you can unplug shelves not in use. But it is more wires to deal with. Easier to replace a single piece if need be. Less Flexwatt used overall.

Both have merits. Pick which you want.
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Mark