I do that also. I even find that it helps me if the outer lines are a skosh wider than the fins. My eyes seem to do a good job of centering the fin between them. These are also good guides if you have to slot the tube.
Manny documented his build at RocketReviews.com:http://www.rocketreviews.com/semroc-cherokee-d-5544.html
BTW ... this is similar to a tip I've posted a couple of times. If you find that a printed fin-wrap guide is a little too small or too large for the body tube, put it on one direction and mark the fin positions. Then, slide the guide off the tube and back on the other direction. Line up one of the marks on the guide with the marks on the body tube. If there's a "launch lug" mark on the guide, line it up with the one on the tube, otherwise line up one of the fin marks. Now, use the guide to draw a second set of marks on the tube. Extend the marks along the tube. You'll have two parallel lines close to each other for some or all of the fins. Just center the roots of the fins between the parallel lines.