[grislybutter, 'Is this true, always? Do I need a sub? If I mostly listen to music with little low frequencies (I think?) in a small room,']
Of course you don't need a sub...or two or eight. Many simply enjoy what the addition of subwoofer/s bring to their systems overall presentation regardless of the main speakers response.
There are manufacturers that offer trial periods and the support to get the system dialed in.
Capable subwoofer systems are a worthy investment. They're an audio component unto themselves that should complement any future speaker or system changes. The phrase, 'matching your subwoofer to your speakers,' can be a red flag.
Hop off that fence. The sound down here is terrific.