Is it worth opening windows in a Denver winter?

Briefly, yes, on a sunny afternoon. Ten or fifteen minutes of cross-ventilation clears a surprising amount of stale air, and the furnace recovers quickly. Close up early enough that the house feels comfortable when buyers arrive.

What about smells from people living in a small space?

Bedding, hampers and gym gear carry more personal odor than the air does. Washing bedding weekly while listed and keeping laundry in closed bins goes a long way.

Showing-Day Odor Reset for Occupied Denver Listings

The reset, by the clock

The evening before: Cook something simple, or eat out. Run the dishwasher. Take trash, recycling and food scraps out to the bins outside the garage if possible.
Morning of: Run bathroom fans after showers for at least twenty minutes. Hang damp towels outside the bathroom or put them in the dryer. Make beds with fresh linens if they are due.
Two hours before: Open windows on two sides of the house for cross-ventilation if the weather allows. Run the HVAC fan.
One hour before: Empty bathroom trash. Check the laundry room for damp clothes in the washer. Wipe the kitchen sink and run the disposal with water.
Thirty minutes before: Close windows in cold or very hot weather so the temperature recovers. Put away gym bags and shoes, which carry more odor than people realize.
Leaving: Step back in through the front door for a final check, then leave.

Quick checks room by room

Kitchen: sink, disposal, dishwasher filter, trash under the sink
Laundry: nothing left damp in the washer, lint trap cleaned
Bathrooms: trash emptied, towels dry, toilet clean, fan off
Bedrooms: hampers emptied or closed in a closet
Entry: shoes and coats put away
Basement: door open, dehumidifier bucket emptied if one is running
Garage: trash bins closed and away from the door into the house

When something goes wrong

IfYou burned food or cooked something strong that morningThenVentilate as long as you can, run the range hood, and let your agent know so they can mention it was cooking, not the house.
IfA drain smells sewer-likeThenRun water into it for a minute; a seldom-used trap may have dried out.
IfThe basement smells musty after rainThenRun a dehumidifier and the HVAC fan, and tell your agent. If it keeps happening, the moisture source needs attention.
IfThe house smells of smoke from a neighbor or wildfire hazeThenKeep windows shut, run the HVAC fan with a good filter, and mention it to your agent.

Common questions

Is it worth opening windows in a Denver winter?: Briefly, yes, on a sunny afternoon. Ten or fifteen minutes of cross-ventilation clears a surprising amount of stale air, and the furnace recovers quickly. Close up early enough that the house feels comfortable when buyers arrive.
What about smells from people living in a small space?: Bedding, hampers and gym gear carry more personal odor than the air does. Washing bedding weekly while listed and keeping laundry in closed bins goes a long way.

General information for the Denver area, not a quote or professional advice. In an emergency, call 911.