The ritual of bringing prickly foliage like holly and pine into a home as decoration during the winter season goes back thousands of years to pagan times, and while the Christmas tree is a relatively new addition, most living trees have sharp needles. These freshly cut boughs and branches may increase tension if left unadorned, but by embellishing them with lights and ornaments you offset much of the cutting 'chi'.
That said, as soon as they start to dehydrate, you’re entering 'dead' energy territory, so make an informed decision on whether to replace or dismantle any dry, drooping greenery before the twelfth night.