I think the point is that people should have sane office hours, that any software firm that default to overtime is up to no good. Also, I don't think having flexible hours is an issue according to 37signals as they have staff located in different countries.

Also, may I suggest that instead of focusing on top-down business principles you should be more product and customer centric and design your dev process around these aspects of your business? If your product or customers require x of you and your staff, then do x, otherwise don't. Deciding on office hours or how many office hours staff should work without a rationale risks creating bureaucracy and a sense of arbitrary decision making.