Opening your fermenter won't mess up anything, so long as you're careful. Yeast metabolize sugar and secrete lots and lots of carbon dioxide (as well as ethanol and trace chemicals called 'esters'). Since carbon dioxide is heavier than air, a layer will form on the surface of your beer. This layer is extremely inhospitable to infecting microflora, and contains little if any oxygen. So don't go sloshing around in your beer and you'll be fine.
Experience will tell you when you should start mucking about. For example, your high-gravity IPA can probably rest a good month in the primary. After the vigorous primary fermentation has completed - which optimally should be within seven days of pitching - the yeast have plenty of time to clean up after themselves.
Let it go another couple of days, to two full weeks from pitching. Then take a gravity reading. Do not return the sample to the fermenter. The risk of contamination outweighs the loss of beer. Wait a day, take another sample. Repeat. When you get three like gravities in three consecutive samples, your ferment is finished. You may feel free to rack, package, dry-hop, whatever.
Cheers,
Bob