Personally, I wouldn't mess with them until the end of day 22. 21 days is a guidline. I think at 21 days they need to be given uninterrupted time. However, like American Mom, I believe that things like this is a personal choice (wether the rest of us agree or not). I would advise that you take one thing into consideration though in making the decision. Your humidity. It is important to have your humidity up for the hatching. If you can keep your humidity up (past the least recommended range) and your bator can quickly get the humidity back up once it's opened or has dropped, then it shoudn't be muchof an issue. However, if you struggle to have high enough humidity or to regain your humidity after opening, I would not open any more than absolutely necessasry. Good luck with the hatch and as hard as it is, try to be patient. I know it's hard not knowing what's going on in there.
Here's something you may consider trying first: If you have a mobile device or computer close enough to the incubator, bring up a youtube video of baby chicks cheeping and play it. (I do this periodically through the day at hatch time and once my first one has pipped.) It motivates them to hear the chirping and if any have internally pipped you may hear it respond or you may see them move to try to pip.
