I use 3/32" when I paint a helmet. Towards the bottom of this thread there's an example of how it looks on a BKBT helmet I painted:
http://www.thedentedhelmet.com/showthread.php?t=32928
When I size up a BKBT's helmet dimensions to ref. pics, I get a little under 1/8" for the pin striping. However, 1/8" is easier to find in most local auto parts stores. I have to special order the 3/32" striping tape I use from our local auto paint store. I might have to try the 1/8" tape one of these times.