This may be a common technique, but does not conform to the National Electrical Code, which requires a transfer device which must prevent the simultaneous connection of generator and utility power. This commonly means a separate breaker subpanel in which you put the circuits you plan on powering during an emergency, and a transfer switch that powers the subpanel with either generator power or utility power, but cannot accidentally have both at the same time. These transfer switches can be automatic, switching to generator power when a power failure is detected.
There are a lot of reasons for this, but an important one is to prevent electrocuting an unsuspecting utility worker who is not expecting power to be coming backwards from the house to the pole when he shows up to re-connect your electricity. Another reason is so that you can more easily control the amount of power draw to your generator (in other words, size your subpanel load to your generator capacity) to avoid damaging your generator.
Again, I realize that backfeeding your panel is a common technique, and in theory, if you are that conscientious person who knows enough about electricity and cares enough to do the right procedures ALL THE TIME, it is an OK practice. However, Murphy is always lurking, and someone can get hurt or killed. We all know what bad electrical practices have done to our fellow servicemembers overseas.
Not to beat a dead horse, but there are a lot of subtleties to electrical work that may not be obvious to the homeowner or amateur electrician. The reasons behind certain codes or procedures may not be apparent, but most of the time, they are for very good reasons.