You will see many posts where a member will ask for opinions on color, layout, etc. I think it is perfectly fine to give an opinion - providing I have one.
Sometimes, members post pictures and they talk about "the flaw" in the top and we generally spend a lot of time finding it. Most times, it is an error that blends so beautifully into the design - without a big pointy stick NOBODY would notice.
Other members post pictures to show off their work. They are pleased with what they have accomplished and (I believe) are looking for general comments from their fellow quilters.
Pointing out an error without invitation to do so would be like bringing out the quilt police. Either the maker DID NOT notice it, and who am I to tattle? OR, he or she did notice it and hopes to heck that the rest of us won't notice. Or it is not important in the greater scheme of things.
IMHO, it is not acceptable to point out an error on a finished quilt.
That said: On occasion, I have seen some works in progress where members expressed frustrations with the executiop. If I know of a way to give advice of how to improve the block - then I will share.
But in the end, we each have to follow our own drummer and do what feels right to us.