The RH setting depends on the quality of the windows and how cold it is outside. A good starting setpoint in the heating season is 35% when it is below freezing out and 50% when it is above freezing out.
Will not be a sweaty window problem when ambient dewpoint is 45F with a ventilated home. Running on a timer could control humidity, running by a dehumidistat will minimize run time especially in the coldest weather.
The quality of the window and the indoor temperature are fixed. The outdoor temperature/dew point are a varible. The inside %RH/dew point must vary with outside temperture to avoid condensation. An outdoor temperature of 32^F may require 40%RH. A -10F outdoor temperature require <25%RH to avoid condensation. In other words, you must re-adjust the indoor %RH to match the changing outside condition. Otherwise the HRV will operate all the time during high outdoor dew points or none of the time during the low dew points.
Combining the fresh air ventilation with time of occupancy is critical regardless of the potiential window problem. This leaves me with the opinion that a timer routinely providing the ventilation while the home is occupied with enough hours to remove the moisture from the occupants is simplest. The resulting inside dew point/%RH will vary with the outside dew point. This provides year around consitant ventilation. Just a difference of opinion. TB