Not really a couple per se, but my best friend, whom I see as a sibling rather than a friend share iTunes. It happened when iTunes Match came into the fold. She has contributed greatly to my iTunes library in the past several years (>14,000 tracks), so I thought that she should have access to it as well. So, basically her device is locked to my AppleID to purchase anything (apps, music, books, etc). If she were to sign out, use her own ID, then sign back in with mine, she would be locked out. Apple now requires devices to be locked to one AppleID for 90 days; there is no way around that.
Cost-wise, it doesn't really matter for me. She can buy whatever she wants and of course, so can I. My account is only filled with iTunes GC, so at times, she refills it by buying an iTunes GC. Been doing this for at least 2-3 years (whenever iTunes Match debuted), and everything works and have never ran into a problem/issue.