As you say, it is common practice for many stressers to only analyze the first load at the PSV in closed systems, often calculating the load via API-520 or similar.

It is typically simultaneously over-conservative and under-conservative. The first run off PSVs is often short. Actual load seen for short runs will be much lower. However, they forgo loads within the rest of the closed system.

I tend to stick with the DLF of 2 throughout a static analysis as this methodology remains both conservative and results in stresses well below allowable in my line of work.