Temporal locality of reference is the case for basically all programs (really oddball scientific ones being the only exception I can think of).
With most servers, though, it doesn't matter. The important bits (the server software) trivially fits into RAM, and the unimportant bits (the served content) comes off disk anyway. In this type of workload, swap is more of a liability than a benefit.
Another way of looking at it: best case it will swap a few unused nginx pages out to disk and bring 0.01% speed improvement, worst case it can fight the buffercache or even bring your server to its knees and light your pager up at 3am.