The song "Tzur MiShelo", generally printed in Bentchers, Siddurim, Zimronim, etc., as a Friday night song, follows the pattern of Bentching (grace after meals), praising G-d and praying for redemption, in the order of the blessings therein.
I have heard some say that you should not sing the song, at least in its entirety, prior to Bentching, because you would fulfill the Biblical requirement of Bentching and reciting the formal Bentching would, in essence, be saying unnecessary blessings, which is prohibited.
However, not everyone practices this stringency, and I'm sure many don't even know about it, and furthermore, someone wrote the song for a reason, right?
Furthermore, I know some communities (I'm using community on the micro-level here to mean groups of people not related who gather for prayers and meals together, not the macro-level of entire synagogues and/or neighborhoods or groups of neighborhoods that share common resources like synagogues, schools, Mikvaoth, etc.) who specifically sing this song as part of their introduction to Bentching. I don't know if this is a widespread custom or limited to those I've encountered who do so, but they are primarily Middle Eastern.
Other than the opinion of the GR"A, cited above, does mainstream thought follow the opinion that you shouldn't recite it prior to Bentching, or do they dismiss that approach (and if the latter, why)?
Is the custom to recite it as an introduction to Bentching based on something? Is it widespread?