We didn't need a monitor until we bought this house when DS was about 3 or 4 months old. At that point I *thought* we were going to be transferring him to his crib, so I wanted the crazy expensive Angel Care monitor - the one that will alarm if baby stops breathing. Well, the crib transfer didn't work, not even for naps at that point, and you actually can't use the angel care thingie without it being plugged into the board that senses baby's breathing (becoming disconnected is one of the reasons why it alarmed). So we couldn't use it. So when DS finally started to be able to be put down for sleeping, I bought the cheapest thing I could find on amazon. I forget the brand now, but it was 20 bucks, and worked like a dream. If you turned the volume up loud you could hear him breathing, and it was super sensitive without having to keep it in/near the crib. DS smashed it at one point, so I got another, "safety first" brand, same deal. The cheapest on amazon at around 20 bucks, and the same deal. I don't *need* it most of the time, but it's great if I need to go out side and do yard work or a project down stairs where I wouldn't be able to hear him. I do use it most days just because I like to know when he's about to wake up so I know I need to wrap up what I am doing - he's never been a schedule oriented kids, so his naps could, and can, be anywhere from 45 min to 3 hours.
I think monitors are a personal preference type of thing unless you would have a problem hearing the baby. They are cheap and easy to get too, so it's definitely one of those things that you can wait and see on. I personally like to hear my son sleeping and when he laughs and talks in his sleep, so even though we perhaps don't *need* one most days in a home our size, I like to use it.