If your referring to "masks",Yeah I use them all the time. My local sign guy cuts them for me. Avery or Gerber are the best vinals to use.Just make sure they are solvent resistant and DON'T leave them on overnight. Pull them as soon as the paint flash's or you will have a BIG gooey MESS to clean off as well as the paint chipping or lifting from the masking edge where the paint has bridged over on to the mask and dried.
My opinion about stencils is they are too "cookie cutter" looking. For the price you'll have to pay, you could go to the paint store and buy 3M blue fine-line tape (the ONLY tape to use for graphic lay-outs). IMO for flames to look good they need to fit the shape of the car. I think repeated flames are the worst. I imagine that if you just start playing around with some tape on the car you'll be able to pull it off. Then you will truly have "custom" flames. Remember that the shape IN the tape is what you'll get not the shape OF the tape. If you try to make the shape of the tape right, you'll end up with flames a little thinner than what you expected.
I like to use at least two colors when I lay-out flames. Lay out the tape. Cover the area you don't want to paint with masking tape. I usually use 2 inch then cut it with a razor blade. Just be careful not to push to hard, as you only want to slice through the masking tape, not the fine-line. I like to spray on the dominant color of the flames, then come back with my hi-light color on the tips and the turn-arounds. Turn-arounds meaning the bottom end of the flames where the tape curls around to head for another tip. to achieve just a hi-light on the turn-arounds, I shoot the masking tape in the middle of the curl and just allow a bit of overspray to fade past the tape and onto the flame. I do not advise hi-lighting all the way around the edge, that usually looks a bit cheesy. Anyway sorry to ramble on, but, I've done alot of flames, kinda tried to steer clear of them, but I do alot of bikes(those guys all seem to want them). Just thought I'd share a few thoughts about it.
Here are some pics of one from a while back. The color combo was all the customers doing. On this one I put on white to cover the purple(yellow don't cover too good) yellow, orange, then red. Then I left the purple all covered and laid some 8th inch tape around the edge and covered everything else, then I pulled off the 8th inch to spray the apple green. It was the first time I've "sprayed" on a pin stripe around the flames. This method gave the pin-stripe nice clean perfect edges, unlike (my imperfect) brush-on pinstripes. Plus, after sanding and buffing you can't feel a thing.