Whether or not to use a subwoofer with monitors is a topic for another thread, but it's worth touching on here.
The main thing to be aware of is that reference-caliber subwoofer systems tend to be expensive and tend to require some significant setup, unlike a home-theater or trunk-mounted thump box. The second thing to be aware of is that subwoofers and very low frequencies in general are not always necessary or desirable for good recordings.
The old RIAA AES mechanical rule for vinyl was to cut at 47Hz and 12k, and some great recordings were made this way. Human perception at extreme highs and lows is not all that accurate or sensitive, and a little goes a long way. If you have accurate monitoring down to say 50 cycles or so, and you simply shelve off everything below that, then you are making recordings that will probably hold up very well in real-world playback on a broad range of systems. The real-world listeners who have the equipment and acoustics to accurately reproduce content below that, and who have the sensitivity to notice it and care are very few and far between.
If you do monitor with subs, make sure the record still sounds good without them.