Using the -s flag might be better than redirecting to /dev/null.

If the servers have 'iburst' 'prefer' etc specified as options, this may currently fail as "iburst" will be specified as a server, for example.  I don't know of another way with the way things are currently, other than using something like:  
    $servers = ['pool.ntp.org iburst', 'time.nist.gov prefer iburst']
etc to specify the options for the servers.