Hi, JMW,
The angle of the spoke holes varies depending on whether the rim was drilled to for a drum or a disc brake (smaller hub). If you lace a big drum brake into a rim drilled for a disc hub, the spokes will take up a bend at the hub end instead of leaving the rim at 90 degrees. It's OK for occasional road use, but the bend introduces extra stress and you could end up popping the heads off the spokes over time. If you use a rim drilled for a drum brake you should be fine.
Graham