The answer is that yes, you can rely on this method to get the last published item, according to the order in the blockchain. If you wanted it according to the order seen by that specific node you would pass true for the local-ordering parameter of liststreamitems but this is not the default behavior.
However please note that it is possible (depending on your miner setup) for there to be a temporary fork in the blockchain, where different nodes see different versions of the latest block, and those two versions have some stream items in a different order. This will resolve quickly, but if you want to make absolutely sure that every node gets the same answer, you should wait for a few confirmations of blocks on top of the item published.