As far as functionality, you probably get the gist of your forearm strength work on back days and bi's day. However, for aesthetic purposes, training the forearms are a good idea, but if your primary concern is functionality, heavy back movements and biceps work should suffice.
I personally train the absolutely hades out of my forearms, but its for aesthetic purposes.
Phil Heath has terrific forearm development, particularly in the brachioradialis region. It adds a thick, strong look that big bi's alone cant' achieve. Although I'm not certain if Phil Heath actually concentrates on forearms, it could be 90% genetics. The only bodybuilder I know that's famed for intensive forearm development is Lee Priest. There are others, but I can't think of them right now.
The bulk of Lee's development are in his flexors. Developed flexors like Lee's offer a little bulkier look, while developed extensors and brachioradialis like Phil's, offers a more muscular look. Which is more appealing to myself, very subjective of course.