Transformer voltage rating vs open-circuit voltage: what's the typical load assumed for the voltage rating?
The nominal output voltage should be the rated voltage with nominal input voltage and the full load (resistive) as rated. 

In other words, a 12V 300mA transformer should have 12V RMS output with nominal input and a 300mA resistive load. For loads less than the rated load, the voltage, of course, will be higher.

Edit:

The regulation of a transformer is typically defined as:

Regulation(%) = \$ \frac {V_{open} - V_{full.load}}{V_{full.load}}\$

Large high power transformers might have regulation of a few percent, cheap small transformers maybe 5 or 10x worse.