Jeff,
The importance of the right forearm depends on various factors:-
1. What type of stroke employed;
2. Which aspects of putting you do well, or badly;
3. How good you are, overall.
Having the right forearm in line with the putter shaft, for me, is most beneficial in promoting the least tension in the arms/shoulders/ wrists.
Some will argue that it helps keep the stroke 'on plane'. I don't personally subscribe to this view. I think that this concept becomes muddled when factoring in shoulder movement, wrist movement, upper spine positioning, and intent, as well as all the potential influences from the putter itself. Of course I am open to other views on this subject, and could not care less if the player does what needs to be done with the roll of the ball.
As for the grip, the more you get the putter grip in the lifelines of each hand, the more that you will match the forearm angles. Palms opposing, thumbs down the center of the grip provide good feedback. Again, the importance depends on you and the above factors.
Damon