If you don't, you'd be sizing a spring without settlement, and when the tank settles, you'll have a sustained load equal to the settlement times spring rate.

The pipe or tank nozzle could fail after settlement. Or it might not fail.

If you do, you'd be sizing a spring that could be insufficient to support the pipe and tank nozzle when it's new. Or it might not fail.

If scenario 1 fails and scenario 2 fails, you might consider adding a half-settlement scenario and size the spring for that, in which case the pipe is only half-under-supported new and half-over-supported with settlement. In which case, it could be almost-failing in both cases.

Ultimately, I would expect that if you have significant settlement, you would end up with a constant effort spring no matter what, should you choose to go spring-support route.

Depending on the situation, you might look into other solutions to enhance flexibility.