You shouldn't lock it open for too long (i.e. over winter) because the extra compression of the spring will weaken it and you could start having leakage at lower line pressure.
I only ever open it when winterizing, after removing the anode rod, for a fast drain. Also when de-winterizing to get a fast fill via the pump or city water (purge the air without having to open a hot tap somewhere).
I've only ever had the pressure relief valve do its intended job when I've completely filled the system with cold water and then had the water heater come up to full temp without opening any tap during the process.
2015 Rockwood Signature UltraLite 8282WS Platinum, GY Marathon LRD, TST 507RV TPMS
2005 GMC 2500HD CCSB D/A, Curt E16, Prodigy P2, Garmin RV760LMT w/BC-20 b/u cam