My question goes to the health of the compressors innards. If it is healthy but it leaks, then you should have a clean system that will not require flushing. If when turning the compressor by the front clutch portion it feels rough or if it was noisy, then you may have debris from the failed compressor in the system to flush out. If that were the case and you didn't flush, then the debris would destroy your new compressor.

If it is only leaking, still turns smooth and made no noise, then you should be good to go without flushing.

I can't remember who the vendors were. It's been a couple of years since I replaced one. The main thing is that these compressors rebuild well as far as I know. There are some others such as the old R4 that should be avoided in rebuilt form.