Yes, because the placard was basically ensuring that he'd be the first to die if someone attacked his place. That's not something you just casually commit to. Especially if you're powerless.
No, it isn't, but that isn't something he wouldn't have done anyway, he didn't need to be asked or to add his blood to seal the deal. As Harry pointed out, Mac knew what the Placard was, what putting his blood on it meant..
Mac rocked back half a pace. He looked from the sign to me. He didn't quite lick his lips in nervous guilt, but it was pretty clear that he didn't like that I'd realize what he knew.
Mac knew what was going to be asked of him, not the same at all as looking after the people in the bar. Mac would have done that anyway,as best he could. The Placard is about more than just that. Mac himmmed and hawaad, looking at the sign, then away..
Choosing to put his blood on the Placard, if it didn't cross the line for Mac, it blurred it to the point where it might no longer be there.. Because;
"What is it?" Murphy asked as he worked.
"The placard from the Cross," I said, "Here is the King of the Jews."
Her golden eyebrows went up. "From the vault?"
"Yeah," I said.
"What does it do?"
"It's embodied intercession. . . It focuses energy on an individual."
I looked up the meaning of "intercession." According to Wikipedia;
Intercession or intercessory prayer is the act of praying to a deity or to a saint in heaven on behalf of oneself or others.
That my friends is taking sides, it is a pledge from Mac, that he'd be willing to ask for divine help to save those in his bar. The guys he refused to back many millennia ago in another battle between good and evil.