Technically, shooting in colour then converting later would save a lot of trouble - unless you shoot RAW
This is especially true when you consider that you can then go over the various colours and pull out certain details you want before the conversion.
That said, if you were to shoot in B&W, it would definitely force you to consider the composition of the image a lot more - without the colour you're forced to use other things to draw the attention of the viewer - it could be a very useful tool to force the way you think - and if you've shot RAW, convert it back if you need to.
No doubt it won't be B&W through the viewfinder, but when you preview your shot, it will be, and whatever things you think you need to do will be there for you to see.
Personally, I love going B&W once in a while - especially when its darker, and the colours are less pronounced. straight to B&W and up the ISO - the grain can help the shot.