My general rule of thumb is to try the simplest method first. In other words don't mess around with the oven until you've satisified yourself that you actually need to.
Also, what I've learned is that sizing may or may not be necessary. For instance, if I just cast a wheel weight bullet and let it air cool, I don't really need to size at all. They chamber just fine, and the powder has no problem swaging them into the bore.
But, if I drop the wheel-weight bullets from the mold directly into a bucket of cold water, I need to size them. This is mainly because when I run the cartridge through the Lee Factory Crimp die, the cases sometimes wrinkle. This is because the die actually re-sizes the case, and the hard bullets pinch the case between the bullet and the die wall. Since the harder bullets don't "give" the case buckles under the pressure. Interestingly the wrinkled cases still chambered and fired.