Well its up to personal preference.Like what jmmtn4aj has said,you can convert and see if you like the pic in B/W or in colour.Usually if i convert a pic to B/W i will still keep a colour copy
btw B/W can to a certain extent hide overexposed or underexposed pics;p
For colours, different colours elicit different emotional responses from the viewer and are all seeking different levels of attention; for e.g. red may suggest "danger" and is screaming for attention while light blue conveys "calm", "cool" or "tranquility" etc.
So there are many things in the photo seeking the attention of the viewer and how much attention they each get depends on the composition.
If colours are taken out of a photo, then the other elements are given more power to catch the viewer's attention and become much more apparent. So black and white photos are good to bring out much more vividly the lines, shapes, form, volume, symmetry, tones and lighting etc. since colours are distracting and may elicit unwanted emotion response from the viewer.
So I b/w whenever the message in my photo is about the elements other than colours.....especially about lines, shapes, form, volume, symmetry, tones and lighting.
I ask myself what was the main thing I wanted to shoot when I took the photo. If it's about lines, shapes, volume, symmetry, tones and lighting, then I would go b/w as this will give the photo the message I wanted and be more likely to elicit the wanted emotional response from the viewer (including myself). So the b/w picture will turn out "nicer" because it is the effect I want.