With my bivy sack and Winter bag putting anything high volume compresses the bag too much. So, the boots spend the night outside usually between pad and toe box of the bivy sack.
NEalpine rules are meant to be broken. Carrying a bag rated 20f colder than what might happen is way to much weight and space. You can easily stretch a 32f bag to about zero, and a 0f bag to -30. All depends on your sleep system. I have no problems sleeping in my puffy and a warm bottle between the legs. Not that I would like to do it for weeks on end but for a couple of nights it's not so bad.
Agreed. Those with experience can get by with a bag rated for exactly what temps they might hit. I am a huge fan of wearing the monster puffy & heavy weight long underwear with a good hat... more comfortable than being perfectly still in a cocoon. A quality sleeping pad goes a LONG way regarding the night's comfort too... My recommendation comes from 20 years of customers expecting a 20 degree bag to keep them toasty warm on a 20 degree night without following all the tricks we learn from our experiences...
1) Eat a high carb/fat meal before bedding down
2) Fill all bottles with boiled water and toss them in the bag while finishing dinner
3) Do a few jumping jacks or other exercise to warm up BEFORE getting in your bag... basically don't get in the sleeping bag when you are already cold.
4) Hydrate, hydrate, hydrate
Getting a bag rated for colder temps helps with the learning curve for a lot of folks new to winter camping...
One of my best purchases ever was a Sea to Summit Reactor Sleeping Bag liner... adds almost no weight or bulk to my kit but adds about 15 degrees of warmth to my sleeping bag (and keeps it clean so I don't have to wash my expensive down bag... ever....)
EDIT: keeping boots in the bivy sack, if you are using one, is definitely sufficient to keep them from freezing, as the temp in the bivy sack will be above freezing.... boots outside of the bivy sack or sleeping bag will freeze solid and cause misery.