Q3Arena.net has posted an interview with id software cartographer Paul Jaquays, that features this bit of advice for dealing with curved surfaces, amongst other topics:

Proceed with caution. They can chew up your poly counts extremely quickly. Use them in grand and obvious ways. Try to keep the curve patch dimensions in even integer multiples of the size of the texture being used (2x, 3x, etc.). That means if a texture is 128x128 pixels, try to make the curve patch be a multiple of 128 (256, 384, etc.).