Do I still need a box spring?

Usually not — but your mattress warranty decides, not fashion. Foam and hybrid mattresses want rigid support: a platform bed or foundation with slats about 3 inches apart or less, or a solid deck. A springy old box spring under memory foam can cause sagging the warranty then excludes. Traditional innerspring mattresses are the exception: some of their warranties still require a matching box spring. Read the warranty card first; it names the exact support it demands.