REVISED for correction and clarity
Your initial question was of spring operated PRVS just before opening. The phenomenon advised is simmer.
Seat tightness is normally the measure at typically a pressure of 90% of set pressure, simulating the normal operating condition. Ie., a 10 % operating gap.
The standard API-527 specifies seat tightness levels.
For all metal to metal seated PRV''s there is an allowable leakage rate at 90% variant by size, pressure..
For soft seats there is no leakage allowance at 90%.
Some companies offer enhanced/better than API-527.
You can expect leakage for spring operated PRV's at 95% metal and O ring seta, regardless what the claims are.
Using soft seats - O Rings, the leakage is minimised. It is more difficult with PTFE. PTFE Seats(which are not O Rings) require some degree of heat to plasticise the seat. Testing at room temperature requires the PRV to be warmed up.
If you are looking for zero leakage at a higher factor then a pilot operated PRV is recommended. The higher the operating pressure the tighter the seats until September point. Pilot PRV''s use O Rings.
What is your specific application and leakage criteria?
Another way is to install a bursting disc upstream of the PRV.
Per ISO-4126, only the term Safety Valve is used regardless of application or design.