I'm thinking along the lines that if toilet cisterns are supplied with cold water taken before PRV, they should not affect taps flow when filling in. For few meters of 15mm pipe that feels like a luxury worth having.

How does it look like in practice?

Presumably this would depend on the differential between the pressure of your cold mains before the PRV and after the PRV. If there is a significant difference (and I have no idea what significant would be) then this could be quite an efficient way of diffusing some pressure that is going to get diffused by the PRV anyway and making use of it to send water to the toilets cisterns more quickly. If your PRV is high up in the property, e.g. loft, then the water will have lost quite a bit of head by the time it gets there, though of course it will gain almost all of that on its way back down after going through the PRV. What stage are you at? Can you take pressure readings before and after PRV so you can work out whether it's worthwhile?

But could a possible downside be that if you have too much pressure taking water to the cisterns your cisterns will be louder?