I have to disagree I'm afraid. All thermostats (despite some commonly held misconceptions) can fail "on" dependent on the component which becomes faulty. This means that they supply full power to the heating element without regulation.
For this reason you should always buy a bulb which has a [i]suitable[/] wattage rating for the size of the enclosure - in the event of a thermostat failure the temperature will rise in the viv, but to an extent that does not kill or permamently damage your reptiles. It may take some experimenting to find the correct wattage for your enclosure, but generally for a 3x2x2 a 100w or 150w should be adequate. A good shop should let you exchange and you can always check this when you purchase.
I would also say that the cost saving between running a correctly rated wattage bulb and a overly-powerful one is less than you may think - the more powerful bulb may run for less of the time, but will draw more current when it does. I certainly don't think the levels of saving are worth potential risk to health.