I think it's a great idea and what everyone should do!
Oh, you wanted only honest responses. Damn.
I might remind the occasional person that I know directly that I did ask for their opinion so they should be open about their reactions and not feel that they have to hold back. But asking someone to be honest is almost a declaration that they would otherwise be dishonest. Even if you happen to think that's true, it's generally considered impolite to actually say it.
As for saying such things on the Internet: I would suggest that, even putting aside the lack of faith implied, it is unnecessary. Most of the anonymous people on the Internet don't seem to have a lot of trouble with honest reviews of someone else's work - sometimes to the extreme. And, added to that, it starts to sound a little desperate.
But, no, I don't want to slap them on the noggin'.