I, personally, think that the juries should vote on the Grand Final night, as the performance should be considered when evaluating the song. Historically, the juries voted on the songs on the night of the Contest. However, the Contest is not, nor should it ever be, a singing competition. The juries are not there to judge vocal abilities (though, Harel was great on the Final night, save for that botched note near the end -- and Didrik, as much as I love that song, was terrible; he was flat throughout the entire song), they are there to judge the songs, their lyrics and music. And, as far as I know, each jury has a certified EBU notary with them during the voting process. The juries are in the host city, watch the dress rehearsals live, then are sequestered, with said notary, until all voting is done. We know the juries are together at some point because each of the past three years, there has been claims that juries have had "inappropriate" contact with one another in their hotel and around the arena (i.e., making deals with people to get them to vote for their country's song).
All things considered, the current voting system is great! I think adopting the Our Sound voting procedure (a separate performance and results show) would be silly and unnecessary.