Is it Safe to Plug an Extension Cord Into a Power Strip?
To be honest, I do it all the time. However, there are some things to keep in mind:


Never exceed the maximum power, current and voltage. 


Voltage will normally not be a problem since this is (more or less) stable in a country. When you go abroad, check if they can be used.
current is most important, sum up all devices you connect on that extension cord AND all devices on extension connected to the the first extension cord.  Take some margin, especially motors sometimes can use a lot of 'starting' current.

Don't forget when you switch on an extension cord (either by plugging it in, or by using a switch if there is on), ALL devices will be powered on at the same time, using possibly some additional extra current.
When you go towards the maximum current supported by the extension cord or in a warm environment or radiation (e.g. from the sun), make sure the cable is fully unrolled (meaning not bundled), since warmth cannot spread (updated according comment of FerryBig).
Apart from heating cables or connectors, a primary safety concern with an extension plugged into an extension of any type is that overall resistivity can become so large that even a full short at the furthest end of the cable may not trip a breaker. Another potential problem is that if the outlet is equipped with an earth-leakage circuit breaker safety device, that may also trip "on it's own" as the current in neutral wire is diminished due to excessive resistivity in the circuit. (addition from Kurja).