I personally use helical because it will put more spin on the arrow during flight. This gives it more stability and better accuracy. If your shop is only fletching your arrows straight and not offset then, you should have them refletched. Straight fletched arrows are simply not as accurate as helical or offset fletched arrows (exception of Quikspin vanes). I use helical because I get better groups with them out of my setup. This may not be the case for you, though. You can experiment and see which gives you better groups.
FOC = Front Of Center as stated above. What that means is the percentage of the arrow in front of excact center of the arrow that is measured from the valley of the nock to the end of the shaft not the insert where the arrow is perfectly balanced with your point, insert, and nock installed. Here's a link that can better explain it.
http://www.socalarchery.com/Information/front%20of%20center.htm
I higher FOC the faster your arrow will stabalize in flight and be on target. I use a FOC of 12% with my arrow, but there arepro's on here that go as high as 25%.