It is possible to get away with just showing one black line through each stripe, unless you have a VERY steady hand and extra fine brush.
The following picture is from an old catalogue of the 'All The Queen's Men' range by Derek Cross (recently retired, though the range now taken over and continuing). I've only shown the one page, as the catalogue has details of most tartans used by British regiments. Anyone wanting specifics has only to drop me a line and I can e.mail the appropriate page.
Finally, a picture of three figures I painted as trials some 20 years ago, when I had a steadier hand. These show three different tartans, with the Black Watch being on the left. I hope the clarity is sufficient, but I took the photo in a hurry about half an hour ago as a lst minute addition.
I must admit that when painting smaller figures I was far more stylistic and didn't follow the detail so avidly. If it looked right I was happy.