How to check on something baking in oven
Yes, clearly steam and heat will escape when you open the oven door.  It is best practice to do this as little as possible during a bake.  Of course, there are many variables, such as ambient room temperature and how long the door is left open, but I have read some estimates that one can lose 150(F) or 66(C) in just a matter of seconds.  To combat this, preheat your oven fully.  For many artisan bread baking formulas, that is often around 475(F) or 246(C) - 500(F) or 260(C). These temperatures are then reduced after steaming. If you are setting up for steam, have those materials in the oven during the preheat.  Place your loaves and water for steam in the oven as quickly as possible.  Don't open the oven until the steaming period is over.  Depending on the recipe, this is typically anywhere from 10 to 20 minutes.  At this point, when most bread recipes call for a reduction in oven temperature anyway, opening the door is less problematic.  Remove the steaming apparatus, close the door, and continue the bake.  Now, nearing the end of your bake, with the bread being almost completed, temperature loss by opening the door probably becomes less important.  The bread is close to completely cooked, so checking near the end should not be too much of a problem. I find that the critical time is during the use of steam and the initial oven spring.

Beyond that, it is probably less critical when dealing with items (such as roast meats or vegetables) which are less sensitive to fluctuations in temperature.