Well, Seth Lover’s breakthrough in inventing the hum-cancelling magnetic pickup for guitars was realizing that besides reversing the winding of the coil on the second bobbin, you had to reverse the polarity of the magnets too (i.e. flip them over).
Other than that, the only big thing I can think of about reversing magnetic polarity is whether you want to chase the little black plastic scotty dog away or catch it with your little white plastic scotty dog.