We use one just when we put her to bed until we go up because sometimes I will be cooking etc downstairs and worry I won't hear her cry... however we sometimes forget to switch it on and I haven't felt that I've ignored her.
If I go to the loo or upstairs for anything else I always poke my head in to check her so don't really know why I use the monitor - probably just because we were given them second hand (I wasn't planning to buy any).
Next week she will move into her own room, and I think I won't bother using them during the night as I feel I probably will react too soon to her stirring when she may just go back to sleep if I leave her.
If you are comfortable, I wouldn't worry about it!