Digital BW, The Print

Perceptual intent squeezes the entire source space to fit within gamut, 
this includes black point. Relative with black point compression 
squeezes the source space to fit with in the black point, and any other 
out of space colors to fit within the gamut. For black and white there 
isn't much difference unless you have a cetain mid gray tone that must 
be reproduced faithfully (not compressed). If you want the full tone of 
the image to fit on the full tone of the paper, perceptual is often the 
best choice (but not always). There really are no hard and fast rules 
for these things, a lot of it has to do with how the image 
looks "best", and it may take a print in each intent to find 
that "best".