From ETS’s guidelines on how institutions should use GRE scores (italics are mine):

Accept Only Official GRE Score Reports
The only official reports of GRE scores are those issued by ETS and sent directly to approved institutions and organizations designated by the test takers and to vendors the score recipients might designate to process the scores they receive. Scores obtained from other sources should not be accepted. If there is a question about the authenticity of a score report, the question should be referred to ETS. ETS will verify whether an official report was issued and the accuracy of the scores.

Why?

Universities already accept unofficial transcripts from applicants. They only collect an official copy when a student enrolls. It’s kind of wild to think that you don’t need verifiable documentation attesting to your institution, degree program, or grades, but you need official test scores.

Don’t require official score reports for applications. It’s expensive, it’s unnecessary, and its one more thing that can arrive late or not be sent or otherwise muck up the admissions timeline. Accept self-reported scores, then gather an official report as a condition of admission.

Update December 2nd, 2018: According to the new remark in the PhD application materials FAQ, you do not have to submit an official report to UW CSE to be fully considered for admission:

A note on self-reported scores: The Allen School will evaluate an application using self-reported scores. However, official scores must be delievered [sic] to the University of Washington prior to accepting an offer of admission.

This was the policy even last year. Consider asking the places you’re applying if they treat unofficial scores the same way.