a real problem with bare-rooting a cactus plant is the likelihood of the roots drying out, as in dieing...
if you receive something bare root, trim the roots back with a scissors; the mere act of trimming them will be to stimulate new root growth if the roots are alive. generally the bare roots will dry & die from the tips back in, so trimming them back is always highly recommended.
regarding when to water, consider this fact: a cactus plant or calloused cutting will grow new roots in bone dry potting soil, searching for water. another fact, cactus plants generally stop growing in the winter, roots included, or they grow much more slowly. warm soil temps promote root growth, cold soil temps inhibit root growth. cold soils and water will lead to more problems than you will want to deal with.
when i root out bare root cuts, most likely they go into 1 gallon pots & they dont get watered for 3 months, or longer. remember, they will grow roots in bone dry soil, so why chance the risk of rotting?