Civil defendants in federal court are obliged to waive service.  All of them, with the notable exception of various governments.  I give you Rule 4(d)(1):

An individual, corporation, or association that is subject to service under Rule 4(e), (f), or (h) has a duty to avoid unnecessary expenses of serving the summons.*

(Emphasis added.)  Seems pretty simple to me. 
Continue Reading Rule 4(d) waivers and fee shifting