You generally want to dither when (before) you quantize, unless you have so much headroom that it doesn't matter. E.g., if you're converting 44.1/16 to 48/16 (which involves quantizing each sample from an intermediate higher-precision result), you probably want to dither, but if you're converting 44.1/24 to 48/24, you probably won't need to care since you don't really care about whether your effective result is 24 or 24.5–25 bits.