...until you've written the whole story.
I'm seeing a trend where writers post their beginning chapter(s) for critique, get feedback, revise, resubmit for critique, get feedback, revise--over and over, you get the idea. What's wrong with that? The beginning is crucial to get right.
I'll tell you what's wrong and this comes from personal experience. If you haven't written down the entire story your polished beginning stands a good chance of getting blown up when you finally have the whole story figured out.
Write the entire draft first. Even better, revise the entire manuscript a couple of times. Then get in and start polishing and seeking feedback. Don't submit an unfinished manuscripts for critique. It's a waste of beta readers' time and a waste of your time. Sure you can learn a lot, but you're story is incomplete. Changes will happen when you finish the rest of it. The voice may change, characters may change, even the entire plot may change. You may find you need to insert foreshadowing in those beginning chapters. Maybe you're world building ideas turn out horribly and you come up with something better by the time you get to the middle of the book. Maybe you've devoted too much effort into a beginning and then find you've run out of word count space for the rest of the book, or you developed polished tangents that have nothing to do with the main thread of the story.
Another danger is that if you do polish up your beginning, you're less likely to want to change it. This can cause a huge struggle with writing the rest. Stories have a tendency to evolve into something different from your first perfect conception. I've seen writers fight the natural flow of their story tooth and nail in order to preserve their precious beginning from needing another overhaul.
And another danger is that if you are submitting an unfinished manuscript over and over to readers to get that beginning just right for agent eyes, you'll wear out your readers. You'll have fewer to turn to when you need help with the middle and ending (which are very important parts too.) By writing and revising the entire manuscript a few times, you'll learn editing skills and catch things on your own before anyone else sees it. You won't catch everything and will still need beta readers at some point, but you'll have fewer mistakes for them to find. The story will flow better from beginning to end and will make more sense for readers.
I know many may ignore this advice. It's too tempting to see if readers like our beginning. We want that gratification of wowing readers. Too soon, my friends. A beautifully crafted beginning is pointless if the rest of the book doesn't have the polished guts to follow. Trust me when I say I'm the voice of experience here (minus the submitting the beginning over and over to readers, in that instance I'm speaking from the position of a beta reader.) Unless you have a complete story down and figured out, it's blowing smoke into a headwind to worry so much over your beginning. So save yourselves some stress, gag that inner need for recognition, and get your whole story written and revised a few times first.