If the images are on the soft side, and you are going dual layer, then it probably won't make any difference if the bitrates are high enough. Past a certain bitrate you stop getting any improvements at all.
In general, a multi pass variable bitrate is best, just don't set the minimum bitrate too low or your quality might suffer.
The problem really occurs where you have detailed images, or a long movie and are going out single layer. Constant bit rates just can't get high enough for the amount data you want to store *and still fit on the disc*, so if going constant, you get ugly problems. (Basically because you need high bitrates for some scenes to look any good at all, but you don't have the space on the disc to do that)
This is where variable bitrates can help, you save space by having a lower bitrate on the scenes that don't really need a high bitrate, and then can use a higher bitrate for the problem scenes, and still fit it onto your disc. Have too low a bitrate on the easy scenes though and they will get ugly, so it is a balancing act.
At work we run an 8 pass variable bitrate, and then look at any potential problem areas and manually change those.