The NSRP parameters preempt and priority are used to control the preferred master.  
Having NSRP preempt properly configured can simplify firewall administration as an administrator will always know which physical firewall is master and which is backup. Please note that if NSRP preemption is configured, one should change the hold-down timer to a higher value (~120-180 seconds) to prevent NSRP failover flapping. Also note that preempt need not be configured on the backup device.
However, note that because NSRP preempt results in an additional NSRP failover event, it should not be configured in environments that seek to minimize NSRP failovers. One such environment is when the NetScreen devices participate in dynamic routing protocols. This is because an NSRP failover event will cause routing protocol reconvergence (a potentially performance-impacting event).