From an ATC point of view, we don't like crossbleed starts on stand as there may be aircraft taxying behind which could possibly get blown over from the increased thrust of the engine used to provide the bleed air to the engine needing to be started.
I've had various power figures quoted to me by different operators / aircraft captains, the highest I've heard was 80% thrust needed to do the crossbleed (which I thought was rather a lot).
So our preference is for the crossbleed to be done after the push with no other aircraft behind.