I have a recollection that this is a requirement in the US (I really ought to know this.......
) but it's certainly the rule of thumb for high density traffic areas such as the Hudson River Corridor (although that might all be about to change).
What is not only a legal requirement in the US but also quite strictly adhered to are the VFR cruising levels. Nobody seems to pay much attention to them on this side of the pond - perhaps due to the position of much of the airspace.