Many subs come with a handy footswitch for bypassing them. If you do get one, make sure it has that feature.
Whether a sub will help or hurt depends a lot on your room. If it's square or cubic and lacks bass traps, a sub will likely complicate things and may not be worth the bother.
Even if you have a large treated room of favorable HxWxL ratios, the sub is still be an optional luxury if your main speakers already extend down to around 45-50 Hz (-3dB). There just isn't much of musical importance happening below that, mostly just unwanted rumble. If somebody else is mixing and mastering, then you needn't care about that. A spectrum analyzer will tell you what you need to know.
If you haven't done any objective measurements on your monitoring environment, now's the time to do that, before you blow any money on a sub. Find out exactly where your speakers wimp out, and where your low-frequency room resonances are. That information will tell you if a sub is needed, optional, or undesirable.
My own situation (probably irrelevant to most folks) is this: I have a medium-quality 4,500 cu. ft. room with just-OK dimensions and inadequate low-frequency acoustic treatments. IOW, it barely qualifies as a suitable environment for a subwoofer. I could do just fine without it, in terms of mixing and mastering. However, I also listen to movies and video games through these speakers, where sonic accuracy isn't as important as just shaking my chair - and the sub's very nice for that.