Most of the curb stencils I see are painted with spray cans. They are usually fuzzy and ugly. When I ask people why they paint them with cans, they say that the striping machine puts out too much paint and they get overspray and runs and the stencil gets too much paint on it.
But spraying with a can causes the spray to leak around the edges of the letters when the stencils don’t fit tightly against the curb and leave the letters fuzzy. It also takes several passes with the can to get coverage. If you don’t hold the can exactly the same way with each pass, the letters get distorted, because the paint is taking a different path through the opening. Also, cans leave an uneven coverage with more paint in one spot than another.
I spray them with my striping machine and get great results, and very little paint build-up. How? Just change tips! Put in a smaller tip like a 4-13 (sometimes called a 1308). It just takes a minute to change tips, and you can paint pretty curb stencils, even if the stencil doesn't fit tightly against the curb. 
You can get an extra tip for $30 and keep it in a butter tub with some paint thinner (or water if all you use is latex). When it’s time to paint small stencils, just pull out the striping tip, drop it in the tub, and replace with the stencil tip. You don’t have to clean them; the solvent in the tub keeps the paint from drying in the tip.
If you are not clear on what the numbers on a tip mean, check out the tip chart in the “Helpful Stuff” section on PLP.