Should instance flush redo log while transferring block in protected read mode in RAC database?

I have general cache fusion question and will see if I can replicate this behavior. Normally when there is GC CR or Current block request from other instances, holding instance (where most current copy of block exists) flushes the redo log before transferring the block. My question is that whether redo log flush is really required in each case. In most of the cases I understand its necessary if block has any uncommitted transactions from other instances, similarly for CR mode also its required because oracle has to apply the undo blocks to generate consistent copy which generated redo and