Does Ripple use NTP or expect the nodes to do so?
If not, why not? It seems that Ripple is a little more closed than your standard cryptocurrency, so this doesn't seem like an excessive demand.
The Ripple server has SNTP support to tolerate system clocks that are inaccurate.
The consensus process also builds a consensus network time. Each validator pulls the consensus network time a bit closer to their local time. This consensus network time becomes the close times included in ledgers that support features like expiring cross-currency offers.