Start by reading the B+W filter handbook: http://dl.dropbox.com/u/8959/b%2Bw_filter_handbook.pdf
At pages 18-19-20-21, you will find the description for their range of KB+KR filters. Notice that their KB filters are equivalent to the Kodak/Wratten series, but for the KR filters, the Kodak/Wratten 81 filters have a different spectral response, and are not equivalent.
You can check the transmission curves at the end of the handbook to understand better their differences.
On page 64-65 you will find a chart to help you figure out which filter you need if you go from situation X and film type Y and want to achieve result Z. These charts are very common, and you can also find them in Kodak Master Photoguides (the blue ones) in the form of a dial calculator. Useful if you have 80-series filters instead of the decamired filters.
In practice: I personally use at least a KR1.5 or 2 filter on all colour shots I take outside, since there is always some blue from the sky washing off the colours. Lately, I've been using a stronger 5 filter for those blazing cloudless days, and on Ektachrome 100G, it was worth it. A stronger filter such as the 11/12 conversion filters is too much for subjects not entirely in the shade.
When inside and shooting with daylight film, remember that even a full conversion filter is not enough to remove the red component of ordinary tungsten bulbs. "Tungsten" refers to photolamps and halogens, and if you want to filter out an off-the-shelf bulb, you will need a full conversion filter + extra filtration.
I personally don't shoot inside much, so I haven't used the blue filters much.