Ok, here are two histograms from the same white target in (almost) the same light:

http://users.dedac.com/cregentin/imgp5863.jpg
http://users.dedac.com/cregentin/imgp5865.jpg

The first is shot with custom WB off the white target itself.

The second is shot with custom WB off a purpleish-pink target.

WB is clearly applied before the raw file.

Whether this is useful to you or not depends on your circumstances, and how much work you want to invest. When shooting in strongly colored light, using custom WB could really improve results vs. fixing the color in post. Additionally, if one makes a habit of shooting strongly colored subjects like flowers or sports jerseys (and has trouble with red-channel clipping), using a lightly colored card to set a custom WB, then applying a compensating color profile in post processing could really improve results - giving you a significant boost in available DR.

For me personally, it's not going to change my life or anything. However, it's nice to have one more trick in the bag for dealing with extraordinary circumstances.