I've unfortunatly seen it before where people would post a piece they've done (drawings usually), and say "Critiques are ok!
" but get upset or defensive when they recieved negative critiques. For whatever reason, they have it in their head that their work is already very good, nothing needs changing, and saying "I want critiques" just makes the compliments they will get that much more fulfilling.
Thankfully, those people are few and far in between.
Generally, I only give critiques if people ask for it. Even so, I try to do the "here's one good thing I like, and here's one thing that needs work." A lot of folks do this as jut a fun hobby, or aren't necessarily ready for a stream of "you should fix this, and this, and this, and..." no matter how nicely it's put. Pointing out the things someone did RIGHT is just as important as pointing out things they could change and try for next time.
If you're a starting artist or customizer, encouragement should always be delivered with critique and advice. Because if all someone hears is what they could change in their work, even if it's not ALL bad, it can be very intimidating.
On the flip side though, people who say flat out "I'm accepting critiques" need to understand what they're agreeing to by saying that. If you don't want critiques, than that should ALSO be said when posting a picture. And if you do say you want them, than it's very important to understand that people posting about errors or fixes or things you could do to make it better aren't saying "Oh, this is a bad custom", they just want to help you to make it the BEST possible custom you can.