One should be careful about waivers of service. It is not an uncommon for an attorney to agree to accept service for a client and then deny having ever done it, thus delaying the action as the plaintiff has to hire a process server or send a pleading through certified mail.
Once the initial pleading is filed, the defendant answers and all parties have attorneys of record in a case, the matter becomes easier as it is sufficient to send subsequent pleadings to the attorney of record. However, that initial pleading should almost always be served on the defendant in a proper manner. Of course, having a valid, signed waiver of service avoids the situation where the plaintiff relies on the good word of the defendant's attorney, but it wouldn't it be easier to simply serve a pleading in the usual manner?