Yes. If you do not sign, you will have to be served.
It is generally a bad idea to sign a waiver of citation of a divorce or custody proceeding in Texas. If you read the fine print, you are not only waiving service of citation, but waiving rights to be served notices of hearings and other actions in the case.
If you wish, you can still waive service of the Petition by simply filing an Answer to the Petition or making an appearance in court. But this type of waiver does not waive other important rights as it would by signing a Waiver of Citation