I'm not sure if the above answers really addressed your question.
For recording vocals in a room, which is what you asked about, Bass Traps are probably NOT necessary, since most human voices don't have a huge amount of energy in the frequency ranges that Bass Traps usually address.
The time when you really do need them is if you are mixing music in that same room. Then the sound and balance of instruments like electric bass, kick drum, or synth bass in the mix will definitely be affected by bass trapping. Depending on your listening position and the size and shape of the room, the perceived low frequencies can often be either doubled or disappear. This makes it hard to judge how much bass is really in your mixes.
By placing bass traps at the intersections of the walls, floor, and ceiling you can help to eliminate that phenomenon, and get a more even and accurate response throughout the room.
For someone doing mixing or mastering, like John (above) it is true that you can't really have too much bass trapping. But, again, if all you are doing is recording vocals in a particular room, I don't think bass traps are a high priority. For vocals in particular, you might be better off with mid to high frequency diffusors.
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