There is a lot of back and forth about using NTP on Amazon's EC2 servers. Here are some links:
A summary then:
Yeah, run NTP. It'll work 80% of the time. It won't work on some other EC2 servers (based on the instance type) but won't error out, a harmless no-op. There is has been reported cases of NTP failing & erroring out when attempting to set the time, but that also seems like a case of outdated kernels (either in the Xen domU or on the guest). So update your VM and terminate/cycle it to hopefully move it do a host Amazon server with the newer kernel.