Both can be used, with a couple of caveats. Pre-cubed squash is super convenient, but sometimes the pieces can be rather small. If you have small slivers in your bag of cubed squash, don't use those, as they'll burn in the oven. Frozen butternut squash will need to be thawed and patted dry before roasting. Otherwise the moisture released will just cause the squash to steam, not roast and caramelize.