Using an AC relay for DC switching, safe limits
No, this is not a general rule. The AC rating and the DC rating rely on different relay qualities.

The AC rating allows for the arc that forms during relay opening to be extinguished by the voltage zero crossing, rather than the opening of the contacts. So the AC rating often says more about how long it can withstand an arc of a certain power level for, rather than how effective it is at interrupting the current.

The DC rating must allow for interruption of the full current at the full voltage rating. So to meet the DC rating, the relay must not only withstand the arc that forms as the contacts are opening, but also open far enough to extinguish that arc.

This is why you see with some relays, and indeed with the relay you specify, that one model is AC rated and another is DC rated - in your case, G5LE-E and G5LE-G respectively. The -G variety actually specifics the contact gap width, because that's important if it's the relay's job to extinguish the arc.

Of course, all relays form some sort of gap, so even an AC rated relay will work with relatively low DC. But how low? It depends. Be especially careful if you have any inductance at all in your load circuit. Interrupting a DC circuit with inductance in it is challenging, and pernicious, because the damage it causes may not be obvious at first.