I use presta on all my bikes just so I don't have to carry two different spare tubes.
I just use a washer that fits around the presta valve stem, bend it to the contours of the inside of the rim and put it under the rim tape. It's never failed me before.
My girlfriend bought a bike and put about 1,000 miles on it before we noticed that the rear rim was drilled for schrader and had presta in it. It probably would've been fine, but a washer costs like 10 cents, so I put a washer on it.