Controlling humidity in your house is important for several reasons. Comfort is one reason, but preventing rot and mold is likely more critical.
In cooler climates moisture must not be allowed to pass through the walls from the inside to the framing and insulation. Humid air will condense in the wall space and create conditions for mold and rot. In extreme cases, insulation can become saturated, or even frozen into a solid block.
The usual solution is to provide a moisture proof barrier under the interior finishing board. This is usually 6 mil polyethylene. Extra care must be taken to seal against all possible leaks. Electrical and plumbing penetrations are common problem areas as well as laps in the poly.
Moisture escaping into the attic can form frost on the underside of the roof. When the weather warms this will melt and can saturate insulation and stain interiors. Good attic ventilation can help to clear moist air that may escape into the space but the poly barrier is the best solution.
Moisture can also condense on inside walls and create an environment for mold. This often happens behind furniture on north walls. Areas with inadequate insulation also create a problem. Forced air heat often does not reach into closets and behind obstructions, hindering drying. Under floor radiant heating is better in this regard. Diagrams of simple radiant systems can be found here.
Too little indoor humidity is not a huge problem but you may be more comfortable if it is maintained at 50 to 60 percent. Older houses with substantial air leakage can have a very dry interior environment in cold weather and a humidifier can add comfort. Too dry air can cause static electricity and dry throats.
Modern houses are normally well sealed and the opposite often occurs. Moisture added from bathrooms, cooking, laundry and even occupant respiration can raise humidity. Attached garages may add humidity from wet cars or snow and ice melting from car undersides. Un-vented gas cook stoves add considerable humidity as a by-product of combustion. Too damp an environment can cause mold growth and windows may frost up in cold weather. Damage can occur when this frost melts. I have even seen door locks freeze solid and become inoperable.
In cold drier climates the best solution is ventilating with outside air. A heat recovery ventilator keeps heat loss to a minimum. This is the method I use in my house. It has the added advantage of replacing stale air with fresh and reducing odors. So far it has been easy to maintain humidity to between 50 and 55 %.
Many contractors will install complicated ventilating systems with many inlets and outlets. I do not think this is necessary and could be very difficult to balance. One or two well placed inlets and one or two outlets should be adequate for most houses. You should also install bathroom exhausts and a kitchen range hood that exhausts to the outside. You may need a separate make up air inlet for these. It is a good idea to wire the bath exhausts to turn on with the light. People will often forget to turn fans on when needed.
Ventilators are usually controlled by a humidistat. You may at times want to control it manually to provide fresh air.
You may need a de-humidifier in humid climates or in especially damp basements. Air conditioners normally reduce indoor humidity in hot climates. Humidity makes hot weather much more uncomfortable. The ability of your body to cool itself by sweating is impaired by high humidity.
Most ventilators available today are probably a little larger than is needed for a smaller house. A smaller unit run more or less continuously may be a better answer. The one in my house is quite large but does not create any real problems except for a slight draft when it is running. Not much of a problem as the moving air is not cold.