The (so called) logic for leaving the loft hatch open is to avoid the potential for the cold water tank freezing. The issue is that with houses having much more insulation in the joists, the cold water tank's temperature is closer to the ambient temperature outside than a few degrees above it.
However, you might not even have a cold water tank in the loft - these days many modern houses don't.
I wouldn't waste heat up to the sky unless you both both the tank AND you leave in an area where the day time temperature regularly keeps below freezing by day - which probably anywhere means north of Newcastle..