I'm gonna be super fucking real with you because I hear a lot of dissent over how to cook rice.
I'm Asian, I was born in SE Asia and I emigrated here when I was two. We first cooked rice in a big-ass pot for a few months because we were too poor to buy a real rice cooker. I eyeballed the rice to water ratio Every. Single. Time. When the rice feels too wet, lift the lid up a bit and let some steam out. If it feels too dry you fucked up and you have to start over. Don't argue with me and tell me to fucking add water because the water doesn't spread evenly over all the rice and creates mushy spots. It's always better to have a little more water that you can let evaporate by lifting the lid.
In regards to proper rice cookers, you can open it to let some steam out. It will not fuck up your rice. More often than not, you're gonna have more water, people usually don't under-water their rice, so more often than not, you're gonna open the lid a bit dry out your rice.
If you eat rice a lot, you should learn how to do two things: eyeball steam and know your cooker's water ratios. Each cooker is different, each cooker uses different heat settings and requires different amounts of water for the rice. KNOW YOUR COOKER. Each time you make a pot of rice, don't use cups to measure the water, use the knuckle of your ring finger as a marker to measure the depth of the water above the rice. Eyeballing steam will let you know how much water you actually let out. When you've cooked rice enough times, you know how much water is in your rice if you mix it up a bit and feel the consistency. Open the lid and stir up the rice a bit (don't fucking burn your hand on the obviously hot steam) and feel the consistency. DOING THIS TOO EARLY IN THE COOKING PROCESS WILL ALWAYS GIVE A SOGGY CONSISTENCY. Wait until all the bubbles have gone and the rice is more or less flat. If it's too soggy, let some steam out. If it's too dry, shut the fuck up, it's not.
TL:DR - Opening the lid is not a sin. Knowing your cooker is tantamount to knowing your water and steam ratios.