Actually if the wiring and breaker are rated at 20 amp and you have 15 amp sockets it is safer, not less safe at least for the home wiring itself. With this setup it is unlikely that a fire could start within the walls. But does leave a possibility of the device plugged in to accept more current then it was designed for, destroying the device or the device itself starting on fire.
A breakdown of what ever you plugged in may not be able to handle the 20 amp and a cord may begin to melt, but long before a fire could occur the 20 amp threshold will pop the circuit. The 5 amp leeway allowed for the initial spike in current many appliances have for that split second when it is turned on.
At least that was the thinking and why the wiring was passed that way at that point in time.
Now the inspectors want 15 amp breakers to match the 15 amp sockets(this protects the plugged in device itself from faltering), but the wiring itself can be greater a gauge , just not a lesser gauge. You can still use 20 amp breaker, 20 amp wring and a 15 amp receptacle though.
In the last 5 years the contractors I have worked with use the cheapest smallest gauge wire allowable. I think the days of overkill are done.
I just pulled a socket. Although the setup on the socket is the traditional 15 amp style the receptacles at my home are heavy duty and can take 20 amps.
Here is the current code, as you can see a 15 amp socket is okay for a 20 amp circuit, circuit meaning breaker and wiring combination:
210.21(B)(1) "Single Receptacle on a Individual Circuit"
A single receptacle installed on a individual branch circuit shall have an apere rating not less than that of the branch circuit.
Can't put a 20 amp receptacle on a 30 amp circuit in this case.
210.21(B)(3) "Receptacle Ratings"
Where connected to a branch circuit suppling two or more receptacles or outlets, receptacle ratings shall conform to the values listed Table 210.21(B)(3)
Circuit Rating = 15 Amps, Receptacle Rating Not over 15 Amps
Circuit Rating = 20 Amps, Receptacle Rating 15 or 20 Amps
Circuit Rating = 30 Amps, Receptacle Rating 30 Amps
Circuit Rating = 40 Amps, Receptacle Rating 40 or 50 Amps
Circuit Rating = 50 Amps, Receptacle Rating 50 amps
Can't put a 20 amp receptacle on a 30 amp circuit here either.
In short a 15 amp receptacle can handle a 20 amp load but a 20 amp receptacle can not handle a 30 amp load.