As the rate of heat loss depends on the temperature differential between outside and inside your best bet is to keep your house as cold as possible for as long as possible. Set it at 5C until you are ready to go home and then bump it up. There are virtually no startup losses as the main form of waste is heat energy leaking out which is a good thing.

There are a million factors to this.  The largest one being your time away from the house.  Without that its really difficult to say. 

Some quick factors that come to mind right away: Amount of insulation, How drafty the house is, Building material of the house (could be factored into the insulation, brick is poor for insulation, where a wood house with proper insulation is far more effective)

If your house is very drafty (enjoy the extremely high heating bill) you might as well just drop it to the lowest temperature you are comfortable with dropping it to.  Becareful about pipes freezing and bursting and what not.  There is absolutely no point in over working your furnace when its just fighting the cold air.

If your house is very well insulated, it really wouldn't matter which way you go with because it just wouldn't drop down that much in the first place, the furnace will just shut off.  Here is where the initial question would matter.  Long periods of time would cool the house, the actual materials which you would have to reheat.  Short periods the house would retain most of its heat, and all you would have to do is reheat the air in the house.   You're going to have to make a reasonable assumption based on the housing matieral's ability to retain heat and your rate of heat loss in the air (drafty or sealed up tight)

Some fun things I've done to save on heating.  During the day open the shades so the sun can shine into the house, I was amazed how well the green house effect worked.

Plastic sheets on the windows, looks kind of tacky, but if you have single pane or drafty windows you really need to.  Being that it gets really cold where you are and I'm assuming window frames are made of aluminum (which has an excellent heat transfer property) the frames will bring a lot of the cold in.  I could be totally wrong about the window frame materials, I haven't spent much time with new windows.

Case and point it really comes down to insulation, and eliminating drafts.

The amount of heat energy that passes through your walls is proportional to the difference in temperature between the inside and outside. If you let the inside cool down more you will expend less energy over the day's cycle than if you keep it at a higher temperature, even if it does take longer to bring it back to a comfortable temperature. This will be far more noticeable with a poorly insulated house.