It depends on what you are using the squash for and the needs of your recipe. If you slice a squash like butternut, acorn or spaghetti squash in half and then place it face down on a baking sheet to roast, you will essentially be steaming the insides. The steam is trapped in the cavity of the squash. This is great if you want the insides soft and spoonable, or if you need to cook it a little faster. Roasting the halves cut-side up cooks the insides without intense steam. You may get some nice caramelization on the surface, and these halves will be a bit more sturdy when tender instead of super soft. You can use that method if you want to stuff the squash halves and serve them whole, or if you'd like to peel the skin and use chunks of squash in a recipe instead of a soft, spoonable situation.
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